Add labels support to Epics

What does this MR do?

This MR adds support for labels in Epics, where user can;

• Add/remove labels to an epic from right sidebar.
• View added labels to an epic in epics list.
• Filter epics list based on labels.
• View Epics list filtered by a Group label directly from Group labels page.

Screenshots

Epics List

Default View	Filter By Label Dropdown	Filtered by Labes

Epic Edit

Default View	Adding a Label	Creating Label Inline

Group Labels

Default View	Mobile View

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 UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• 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?

Closes #3727 (closed), #4032 (closed)