Extract admin strings

Adds app.trans calls for strings used by the admin UI.
- Strings for AddExtensionModal.js not included.
- Corresponding YAML will be sent later w/ more extracted strings.

js/admin/src/components/LoadingModal.js

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