Enable labels for epics

What does this MR do?

There is also CE-only part of this patch - gitlab-ce!17209

Adds support for:

• filtering epics by labels
• adding and removing labels to epics

List of available labels is taken from epic's group and all subgroups.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#3727 (closed)