Refactor EpicHeader to use IssuableHeader

...s/vue_shared/issuable/show/components/issuable_header.vue

ee/app/assets/javascripts/epic/components/epic_header.vue

...ons/jira/issues_show/components/jira_issues_show_root.vue

...zentao/issues_show/components/zentao_issues_show_root.vue

ee/spec/features/epics/epic_show_spec.rb

ee/spec/frontend/epic/components/epic_header_spec.js

...e_shared/issuable/show/components/issuable_header_spec.js