Updated type attribute for dropdown button

no refs

The dropdown button component was missing `type="button"` when used as a button, which caused it to act like a `submit` button when used as part of a form, like in Member details page. This change adds type attribute to the button to fix the issue.

app/components/gh-dropdown-button.js

@@ -1,17 +1,22 @@
 import Component from '@ember/component';
 import DropdownMixin from 'ghost-admin/mixins/dropdown-mixin';
+import {computed} from '@ember/object';
 import {inject as service} from '@ember/service';
 
 export default Component.extend(DropdownMixin, {
     dropdown: service(),
 
     tagName: 'button',
-    attributeBindings: ['href', 'role'],
+    attributeBindings: ['href', 'role', 'type'],
     role: 'button',
 
     // matches with the dropdown this button toggles
     dropdownName: null,
 
+    type: computed(function () {
+        return this.tagName === 'button' ? 'button' : null;
+    }),
+
     // Notify dropdown service this dropdown should be toggled
     click(event) {
         this._super(event);