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/Modal.js

@@ -29,7 +29,7 @@ export default class Modal extends Component {
           {this.isDismissible() ? (
             <div className="Modal-close App-backControl">
               {Button.component({
-                icon: 'times',
+                icon: 'fa fa-times',
                 onclick: this.hide.bind(this),
                 className: 'Button Button--icon Button--link'
               })}