Add third tier to key namespacing

- Changes all `app.trans` calls to `app.translator.trans` calls.
- Changes existing keys to [three-tier namespace structure](https://github.com/flarum/english/pull/12).
- Extracts additional strings for `lib:` namespace.
- Extracts two previously missed strings for EditGroupModal.js.

js/forum/src/components/DiscussionRenamedNotification.js

@@ -20,6 +20,6 @@ export default class DiscussionRenamedNotification extends Notification {
   }
 
   content() {
-    return app.trans('core.forum.notifications_discussion_renamed_text', {user: this.props.notification.sender()});
+    return app.translator.trans('core.forum.notifications.discussion_renamed_text', {user: this.props.notification.sender()});
   }
 }