FontAwesome v5.0.6 (#1372)

* Update FontAwesome to v5.0.6

* Adapt DiscussionListItem-count icon to match FontAwesome 5 syntax

* Change icon name to match FontAwesome 5.0.6 fas icon

* Add font type prefix parameter to icon helper

* Add Enable Icon Prefix to show icon in Extension Page

* Fix invalid icon behavior

* Change icon name to match FontAwesome 5.0.6 far icon

* Use iconPrefix property on component

* Use full icon class name

* Update icon helper docblock

* Full icon class syntax

js/lib/components/SplitDropdown.js

@@ -28,7 +28,7 @@ export default class SplitDropdown extends Dropdown {
         className={'Dropdown-toggle Button Button--icon ' + this.props.buttonClassName}
         data-toggle="dropdown">
         {icon(this.props.icon, {className: 'Button-icon'})}
-        {icon('caret-down', {className: 'Button-caret'})}
+        {icon('fa fa-caret-down', {className: 'Button-caret'})}
       </button>
     ];
   }