migrate admin:* to static bundle

app/assets/javascripts/dispatcher.js

@@ -278,9 +278,6 @@ var Dispatcher;
       }
       switch (path[0]) {
         case 'admin':
-          import('./pages/admin')
-            .then(callDefault)
-            .catch(fail);
           switch (path[1]) {
             case 'broadcast_messages':
               import('./pages/admin/broadcast_messages')

app/assets/javascripts/pages/admin/index.js

 import initAdmin from './admin';
 
-export default () => initAdmin();
+document.addEventListener('DOMContentLoaded', initAdmin);