Merge branch 'ee-5514-fix-epic-collapsed-sidebar-label-click' into 'master'

CE Backport: Reveal labels dropdown when labels icon is clicked on collapsed Epic sidebar

See merge request gitlab-org/gitlab-ce!18611

app/assets/javascripts/vue_shared/models/label.js

-class ListLabel {
+export default class ListLabel {
   constructor(obj) {
     this.id = obj.id;
     this.title = obj.title;