feat: frontend Model extender (#3646)

* feat: reintroduce frontend extenders
* chore: used `Routes` extender in bundled extensions
* chore: used `PostTypes` extender in bundled extensions
* chore: `yarn format`
* feat: `Model` frontend extender
* chore: naming
* chore(review): attributes can be nullable or undefined
* chore(review): delay extender implementation
* chore(review): unnecessary check
* chore(review): stay consistent
* chore: merge conflicts
* chore: unused import
* chore: multiline extenders
* feat: add Store extender

Signed-off-by: Sami Mazouz <sychocouldy@gmail.com>

js/src/forum/extend.ts

@@ -1,4 +1,11 @@
 import Extend from 'flarum/common/extenders';
 import IndexPage from 'flarum/forum/components/IndexPage';
+import Discussion from 'flarum/common/models/Discussion';
 
-export default [new Extend.Routes().add('following', '/following', IndexPage)];
+export default [
+  new Extend.Routes() //
+    .add('following', '/following', IndexPage),
+
+  new Extend.Model(Discussion) //
+    .attribute('subscription'),
+];

js/src/forum/index.js

@@ -16,8 +16,6 @@ export { default as extend } from './extend';
 app.initializers.add('subscriptions', function () {
   app.notificationComponents.newPost = NewPostNotification;
 
-  Discussion.prototype.subscription = Model.attribute('subscription');
-
   addSubscriptionBadge();
   addSubscriptionControls();
   addSubscriptionFilter();