Merge branch 'migrating-event-hub-to-mitt' into 'master'

Migrating  from Vue event hub to mitt

See merge request gitlab-org/gitlab!41160

app/assets/javascripts/sidebar/event_hub.js

-import Vue from 'vue';
+import createEventHub from '~/helpers/event_hub_factory';
 
-const eventHub = new Vue();
+const eventHub = createEventHub();
 
 // TODO: remove eventHub hack after code splitting refactor
 window.emitSidebarEvent = (...args) => eventHub.$emit(...args);