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/Alert.js

@@ -37,7 +37,7 @@ export default class Alert extends Component {
     if (dismissible || dismissible === undefined) {
       dismissControl.push(
         <Button
-          icon="times"
+          icon="fa fa-times"
           className="Button Button--link Button--icon Alert-dismiss"
           onclick={ondismiss}/>
       );