Quick action to add/remove issue to epic from issue

What does this MR do?

It adds the quick actions for assigning an epic to an issue (from the issue) and for removing an epic from an issue (again from the issue).

Commands were added to the EE version of QuickActions:InterpretService. Both actions check if epics are enabled for the group an issue is under (that means these actions are not supported on issues in user namespace projects).

The actions called Issue::UpdateService because they are called from issues. We don't have assining/unassigning epics to issues in this service, we user EpicIssues::CreateService and EpicIssues::DestroyService for this. Therefore I had to add handling epic into the update service (EE version).

Only full references (eg. gitlab-org&1) and full urls are supported. Autocomplete is not part of this issue.

Examples:

/epic gitlab-org&304
/epic https://gitlab.com/groups/gitlab-org/-/epics/304
/remove_epic

What are the relevant issue numbers?

#6959 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #6959 (closed)