Backport CE changes from "autocomplete usernames in Epic comments/description"

What does this MR do?

Backport CE changes in gitlab-org/gitlab-ee!5475

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Squashed related commits together

What are the relevant issue numbers?

gitlab-org/gitlab-ee#4084