Improve extensions page

- Allow extensions to define an icon in their flarum.json
- Show a "please wait" modal when enabling/disabling an extension
- Styling tweaks

js/lib/components/ModalManager.js

@@ -50,7 +50,7 @@ export default class ModalManager extends Component {
 
     m.redraw(true);
 
-    this.$().modal('show');
+    this.$().modal({backdrop: this.component.isDismissible() ? true : 'static'}).modal('show');
     this.onready();
   }