View subgroup epics in epics list page

• We should view all subgroups' (all the way nesting down) epics in the epics list page.
  • Similar to how issues should look on the various dashboard/group/project list pages. I.e. they should just be a flat list as the first iteration. And you can see from the path which project/group it belongs to.
• The epics list reference ID's should be relative to the current group. Example:
  • Current group: &1
  • Subgroup nested 1 level: subgroup&1
  • Subgroup nested 2 levels: subgroup/deeeeeep&1
• We should have the same API support, per discussion as documented here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4182#note_56131942.