Refactor dispatcher milestones show path

app/assets/javascripts/dispatcher.js

 /* eslint-disable func-names, space-before-function-paren, no-var, prefer-arrow-callback, wrap-iife, no-shadow, consistent-return, one-var, one-var-declaration-per-line, camelcase, default-case, no-new, quotes, no-duplicate-case, no-case-declarations, no-fallthrough, max-len */
-import Milestone from './milestone';
 import notificationsDropdown from './notifications_dropdown';
 import LineHighlighter from './line_highlighter';
 import MergeRequest from './merge_request';
-import Sidebar from './right_sidebar';
 import Flash from './flash';
 import BlobViewer from './blob/viewer/index';
 import GfmAutoComplete from './gfm_auto_complete';
@@ -92,9 +90,14 @@ import SearchAutocomplete from './search_autocomplete';
             .catch(fail);
           break;
         case 'projects:milestones:show':
+          import('./pages/projects/milestones/show')
+            .then(callDefault)
+            .catch(fail);
+          break;
         case 'groups:milestones:show':
-          new Milestone();
-          new Sidebar();
+          import('./pages/groups/milestones/show')
+            .then(callDefault)
+            .catch(fail);
           break;
         case 'dashboard:milestones:show':
           import('./pages/dashboard/milestones/show')

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

+import initMilestonesShow from '~/pages/init_milestones_show';
+
+export default initMilestonesShow;

app/assets/javascripts/pages/init_milestones_show.js

+/* eslint-disable no-new */
+
+import Milestone from '~/milestone';
+import Sidebar from '~/right_sidebar';
+
+export default () => {
+  new Milestone();
+  new Sidebar();
+};

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

+import initMilestonesShow from '~/pages/init_milestones_show';
+
+export default initMilestonesShow;