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/admin/src/components/LoadingModal.js

@@ -10,7 +10,7 @@ export default class LoadingModal extends Modal {
   }
 
   title() {
-    return app.trans('core.admin.extensions_loading_title');
+    return app.translator.trans('core.admin.loading.title');
   }
 
   content() {