Autocomplete does not work when editing an epic

Autocomplete does not work when editing an epic. The dropdown shows up but then a flash message says that an error has occurred.

Requirements

We should just disable autocomplete completely in epics. So this includes:

• Emoji

• @-people

• issues

• merge requests

• labels

• milestones

• All of these should not be possible as autocomplete (with the dropdown) in the epic description.

• https://gitlab.com/gitlab-org/gitlab-ee/issues/4084 addresses how to do autocomplete for epics.

• https://gitlab.com/gitlab-org/gitlab-ee/issues/3889 brings the comment thread to epics. Likely #3889 (closed) will be done before we solve autocomplete. So that means the comment thread when it's introduced also will not have autocomplete. When #4084 (closed) is designed and released, it should apply for both epic description and epic comments.