Initialise component state in init() instead of constructor

This allows component state to be overridden via monkey-patch. ref #246

js/forum/src/components/NotificationList.js

@@ -9,9 +9,7 @@ import Discussion from 'flarum/models/Discussion';
  * notifications, grouped by discussion.
  */
 export default class NotificationList extends Component {
-  constructor(...args) {
-    super(...args);
-
+  init() {
     /**
      * Whether or not the notifications are loading.
      *