View containing epic in issue

• View containing epic in issue.
• Navigate to the containing epic in issue.
• The new section in the sidebar should only appear if the epic's group is in EEU (for GitLab.com), or the self-hosted instance has a EEU license.
• Also for subgroups of a EEU group would work the same since subgroups of a EEU group have EEU functionality. That is, that new section would appear in the case that epic is in that subgroup.

Out of scope

• In the future, we may consider assigning an issue to an epic from the issue itself. That is not in scope here. So therefore, it's just a link.

Edge cases

• None so far

Designs

Expanded (filled)	Expanded (empty)	Collapsed

• It should use the epics icon from the navigation sidebar.
• If there's no associated epic, the area should just say None.
• The tooltip contents to show the full epic title
  • If there's no assigned epic, we just show “Epic” in the tooltip.