Epics autocomplete username

• Follow up of https://gitlab.com/gitlab-org/gitlab-ee/issues/4066.
• In #4066 (closed), we are disabling autocomplete in epics because of the problems mentioned there.
• This issue solves those problems so that we can re-introduce autocomplete for epics.
• Autocomplete should work when you type in the epic description and when you type an epic comment.
• When opening the username autocomplete dropdown, the behavior should be the same as in issues and merge requests.
• When autocompleting usernames, search the following:
  • All members in the current group.
  • All members in subgroups.
  • All members in projects in all those groups/subgroups.