autocomplete usernames in Epic comments/description

What does this MR do?

• Enables autocomplete for Users inside Epics
• Fixes participants for Epics

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Squashed related commits together
• Backport MR on CE repo

What are the relevant issue numbers?

Closes #4084 (closed)