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/lib/helpers/username.js

@@ -6,7 +6,7 @@
  * @return {Object}
  */
 export default function username(user) {
-  const name = (user && user.username()) || app.trans('core.forum.user_deleted_text');
+  const name = (user && user.username()) || app.translator.trans('core.lib.deleted_user_text');
 
   return <span className="username">{name}</span>;
 }