Basic working module created.

Block/FacebookChat.php

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+<?php
+
+namespace Newbury\FacebookChat\Block;
+
+use Magento\Framework\View\Element\Template;
+use Magento\Framework\App\Config\ScopeConfigInterface;;
+use Magento\Framework\Locale\Resolver;
+
+class FacebookChat extends Template
+{
+    const CONFIG_PATH_PAGE_ID = 'newbury/facebookchat/page_id';
+
+    /**
+     * @var ScopeConfigInterface
+     */
+    protected $config;
+
+    /**
+     * @var Resolver
+     */
+    protected $localeResolver;
+
+    /**
+     * FacebookChat constructor.
+     * @param Template\Context $context
+     * @param ScopeConfigInterface $config
+     * @param Resolver $resolver
+     * @param array $data
+     */
+    public function __construct(
+        Template\Context $context,
+        ScopeConfigInterface $config,
+        Resolver $resolver,
+        array $data = []
+    ) {
+        parent::__construct($context, $data);
+        $this->config = $config;
+        $this->localeResolver = $resolver;
+    }
+
+    /**
+     * @return mixed
+     */
+    public function getPageId()
+    {
+        return $this->config->getValue(self::CONFIG_PATH_PAGE_ID);
+    }
+
+    /**
+     * @return string
+     */
+    public function getLocale()
+    {
+        return $this->localeResolver->getLocale();
+    }
+}

LICENSE

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+MIT License
+
+Copyright (c) 2018 Craig Newbury
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

composer.json

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+{
+  "name": "newbury/module-facebook-chat",
+  "description": "Magento 2 module allowing the easy installation of the Facebook customer chat plugin.",
+  "require": {
+    "php": "7.0.2|7.0.4|~7.0.6|~7.1.0"
+  },
+  "type": "magento2-module",
+  "version": "1.0.0",
+  "license": [
+    "MIT"
+  ],
+  "autoload": {
+    "files": [
+      "registration.php"
+    ],
+    "psr-4": {
+      "Newbury\\FacebookChat\\": ""
+    }
+  },
+  "authors": [
+    {
+      "name": "Craig Newbury",
+      "email": "[email protected]",
+      "role": "Developer"
+    }
+  ],
+  "support": {
+    "email": "[email protected]"
+  }
+}

etc/acl.xml

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+<?xml version="1.0" ?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Acl/etc/acl.xsd">
+    <acl>
+        <resources>
+            <resource id="Magento_Backend::admin">
+                <resource id="Magento_Backend::stores">
+                    <resource id="Magento_Backend::stores_settings">
+                        <resource id="Magento_Config::config">
+                            <resource id="Newbury_Modules::group" title="Magento modules by Craig Newbury">
+                                <resource id="Newbury_FacebookChat::config" title="Facebook Chat Configuration"/>
+                            </resource>
+                        </resource>
+                    </resource>
+                </resource>
+            </resource>
+        </resources>
+    </acl>
+</config>

etc/adminhtml/system.xml

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+<?xml version="1.0" ?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
+    <system>
+        <tab id="newbury" sortOrder="500" translate="label">
+            <label>Newbury Modules</label>
+        </tab>
+        <section id="newbury_facebookchat" showInDefault="1" showInStore="1" showInWebsite="1" sortOrder="10" translate="label">
+            <label>Facebook Chat Configuration</label>
+            <tab>newbury</tab>
+            <resource>Newbury_FacebookChat::config</resource>
+            <group id="config" showInDefault="1" showInStore="1" showInWebsite="1" sortOrder="10" translate="label">
+                <label>Facebook Chat Settings</label>
+                <field id="enabled" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
+                    <label>Enabled</label>
+                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
+                    <config_path>newbury/facebookchat/enabled</config_path>
+                </field>
+
+                <field id="page_id" translate="label" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
+                    <label>Page ID</label>
+                    <config_path>newbury/facebookchat/page_id</config_path>
+                </field>
+            </group>
+        </section>
+    </system>
+</config>

etc/config.xml

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+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
+    <default>
+        <newbury>
+            <facebookchat>
+                <enabled>0</enabled>
+            </facebookchat>
+        </newbury>
+    </default>
+</config>

etc/module.xml

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+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
+    <module name="Newbury_FacebookChat" setup_version="1.0.0"/>
+</config>

registration.php

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+<?php
+\Magento\Framework\Component\ComponentRegistrar::register(
+    \Magento\Framework\Component\ComponentRegistrar::MODULE,
+    'Newbury_FacebookChat',
+    __DIR__
+);

view/frontend/layout/default.xml

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+<?xml version="1.0"?>
+<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
+    <body>
+        <referenceContainer name="before.body.end">
+            <block class="Newbury\FacebookChat\Block\FacebookChat"
+                   template="Newbury_FacebookChat::FacebookChat.phtml"
+                   name="newbury_facebookchat_chatcode"
+                   after="-"
+                   ifconfig="newbury/facebookchat/enabled" />
+        </referenceContainer>
+    </body>
+</page>

view/frontend/templates/FacebookChat.phtml

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+<?php /**  @var $block \Newbury\FacebookChat\Block\FacebookChat */ ?>
+<?php if(empty($block->getPageId())) return; ?>
+<script>
+    (function(d, s, id) {
+        var js, fjs = d.getElementsByTagName(s)[0];
+        if (d.getElementById(id)) return;
+        js = d.createElement(s); js.id = id;
+        js.src = 'https://connect.facebook.net/<?= $block->getLocale() ?>/sdk/xfbml.customerchat.js#xfbml=1&version=v2.12&autoLogAppEvents=1';
+        fjs.parentNode.insertBefore(js, fjs);
+    }(document, 'script', 'facebook-jssdk'));
+</script>
+<div class="fb-customerchat"
+     page_id="<?= $block->getPageId() ?>">
+</div>