CE Backport: Add support for autocompleting Epics within Issues & MRs and Labels within Epics

What does this MR do?

This MR has changes backported from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6195.

Does this MR meet the acceptance criteria?

• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5604, https://gitlab.com/gitlab-org/gitlab-ee/issues/5605