send an email on account approval

app/controllers/settings/users_controller.rb

@@ -44,6 +44,7 @@ def update
       redirect_to [:settings, @user], notice: t(".reset_link_sent")
     elsif params[:approve]
       @user.update(approved: true)
+      UserMailer.with(user: @user).account_approved.deliver_later if SiteSettings.email_configured?
       redirect_to [:settings, @user], notice: t(".approved")
     elsif @user.update(user_params)
       redirect_to [:settings, @user], notice: t(".success")

app/mailers/user_mailer.rb

@@ -0,0 +1,6 @@
+class UserMailer < ApplicationMailer
+  def account_approved
+    @user = params[:user]
+    mail to: @user.email
+  end
+end

app/views/user_mailer/account_approved.text.erb

@@ -0,0 +1,3 @@
+<%= translate(".greeting") %>
+
+<%= translate(".message", link: new_user_session_url) %>

config/locales/en.yml

@@ -394,6 +394,11 @@ en:
     myminifactory: MyMiniFactory
     theminiindex: The Mini Index
     thingiverse: Thingiverse
+  user_mailer:
+    account_approved:
+      subject: Account approved
+      greeting: Hi!
+      message: Your account has been approved; you may now sign in at %{link}.
   views:
     pagination:
       first: "« First"

spec/mailers/user_mailer_spec.rb

@@ -0,0 +1,20 @@
+require "rails_helper"
+
+RSpec.describe UserMailer do
+  describe "account_approved" do
+    let(:user) { create(:user) }
+    let(:mail) { described_class.with(user: user).account_approved }
+
+    it "sets correct subject" do
+      expect(mail.subject).to eq("Account approved")
+    end
+
+    it "sends to user" do
+      expect(mail.to).to eq([user.email])
+    end
+
+    it "renders the body" do
+      expect(mail.body.encoded).to match("Hi")
+    end
+  end
+end