refactor: replace extend with beforeMount

Author: rafaucau

js/src/forum/index.ts

@@ -1,14 +1,10 @@
-import Application from 'flarum/common/Application';
-import { extend } from 'flarum/common/extend';
 import app from 'flarum/forum/app';
 import MobileTab from './components/MobileTab';
 
 export { default as extend } from './extend';
 
 app.initializers.add('acpl/mobile-tab', () => {
-  //@ts-ignore - missing 'mount' types
-  extend(Application.prototype, 'mount', () => {
-    const mTab = document.createElement('div');
-    m.mount(document.body.appendChild(mTab), MobileTab);
+  app.beforeMount(() => {
+    m.mount(document.body.appendChild(document.createElement('div')), MobileTab);
   });
 });