migrate dashboard:milestones:show to static bundle

app/assets/javascripts/dispatcher.js

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

app/assets/javascripts/pages/dashboard/milestones/show/index.js

 import Milestone from '~/milestone';
 import Sidebar from '~/right_sidebar';
 
-export default () => {
+document.addEventListener('DOMContentLoaded', () => {
   new Milestone(); // eslint-disable-line no-new
   new Sidebar(); // eslint-disable-line no-new
-};
+});