Epic select in issue sidebar doesn't include ancestor group Epics
From https://gitlab.com/gitlab-org/gitlab-ee/issues/6878#note_209589123
Epics are allowed to have descendant group issues so we should allow selection of ancestor group Epics in the issue sidebar.
This would probably mean supporting a include_ancestor_groups
flag in the Epics API that we use and changing the frontend to make the request with that flag.