refactor the fa-spinner for notifications_dropdown.js

app/assets/javascripts/notifications_dropdown.js

@@ -15,7 +15,7 @@ export default function notificationsDropdown() {
       .parents('.notification-form')
       .first();
 
-    form.find('.js-notification-loading').toggleClass('fa-bell fa-spin fa-spinner');
+    form.find('.js-notification-loading').toggleClass('spinner');
     if (form.hasClass('no-label')) {
       form.find('.js-notification-loading').toggleClass('hidden');
       form.find('.js-notifications-icon').toggleClass('hidden');