Support mentioning epics

TODO

• CE port (will do after at least the first review round)

WIP

• Move EE-only code to ee dir/mixins
• Specs
  • Test reference prefix (fail when existing prefix is being added)
  • Add tests for EpicReferenceParser
  • Check another specs should be added
• Check banzai filters for another duplications that could be removed
  • remove duplications: merge_requests_per_project, issues_per_project, epics_per_project
• Add documentation

What does this MR do?

It adds support for mentioning epics.

Formats:

• Shorhand: &epic_id
• Full cross reference: group.full_path&epic.id
• Full url

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We want to support mentioning epics

Screenshots (if relevant)

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

What are the relevant issue numbers?

Closes #3853 (closed)