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/Select.js

@@ -18,7 +18,7 @@ export default class Select extends Component {
         <select className="Select-input FormControl" onchange={onchange ? m.withAttr('value', onchange.bind(this)) : undefined} value={value}>
           {Object.keys(options).map(key => <option value={key}>{options[key]}</option>)}
         </select>
-        {icon('sort', {className: 'Select-caret'})}
+        {icon('fa fa-sort', {className: 'Select-caret'})}
       </span>
     );
   }