Add support for autocompleting Epics and Labels within Epics

The backport to CE is here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20419

What does this MR do?

Enable autocomplete for epic, under epic description and epic comment textareas. Note that this has been scoped down, so epics from other groups are not listed in the drop down for now.

Why was this MR needed?

Allow referencing Epic in another epic easier.

Screenshots

Epics Autocomplete

Labels Autocomplete

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• 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
• 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 #5604 (closed), #5605 (closed)