Merge branch 'migrate-js-events-hub-to-mitt' into 'master'

Migrate event hub to mitt in boards, roadmap, and notes

Closes #216809, #216810, and #216812

See merge request gitlab-org/gitlab!37737

app/assets/javascripts/boards/eventhub.js

-import Vue from 'vue';
+import createEventHub from '~/helpers/event_hub_factory';
 
-export default new Vue();
+export default createEventHub();

app/assets/javascripts/notes/event_hub.js

-import Vue from 'vue';
+import createEventHub from '~/helpers/event_hub_factory';
 
-export default new Vue();
+export default createEventHub();

ee/app/assets/javascripts/roadmap/event_hub.js

-import Vue from 'vue';
+import createEventHub from '~/helpers/event_hub_factory';
 
-export default new Vue();
+export default createEventHub();