migrate explore:groups:index to static bundle

app/assets/javascripts/dispatcher.js

@@ -53,11 +53,6 @@ var Dispatcher;
         case 'projects:issues:show':
           shortcut_handler = true;
           break;
-        case 'explore:groups:index':
-          import('./pages/explore/groups')
-            .then(callDefault)
-            .catch(fail);
-          break;
         case 'projects:milestones:new':
         case 'projects:milestones:create':
           import('./pages/projects/milestones/new')

app/assets/javascripts/pages/explore/groups/index.js

@@ -2,7 +2,7 @@ import GroupsList from '~/groups_list';
 import Landing from '~/landing';
 import initGroupsList from '../../../groups';
 
-export default function () {
+document.addEventListener('DOMContentLoaded', () => {
   new GroupsList(); // eslint-disable-line no-new
   initGroupsList();
   const landingElement = document.querySelector('.js-explore-groups-landing');
@@ -13,4 +13,4 @@ export default function () {
     'explore_groups_landing_dismissed',
   );
   exploreGroupsLanding.toggle();
-}
+});