[WIP] JS Extender API foundation (#1468)

* Run extenders exported by extensions
* Add some basic extenders
* Patch Mithril as the very first thing so extension code can run safely
* Load the payload into the app before booting extensions
* Setup default routes before booting extensions

js/src/forum/ForumApplication.js

@@ -63,13 +63,10 @@ export default class ForumApplication extends Application {
    */
   history = new History();
 
-  /**
-   * @inheritdoc
-   */
-  boot(data) {
-    routes(this);
+  constructor() {
+    super();
 
-    super.boot(data);
+    routes(this);
   }
 
   /**