Typing @ then [TAB] in a comment doesn't select the author

@DouweM and I (and, I hope, other people) do this a lot:

1. Review an MR.
2. Add some comments.
3. Assign back to the author by writing a comment, and in that comment field, typing /ass followed by TAB TAB. This has the effect of autocompleting the /assign slash command and inserting an @ symbol, then using the @ symbol to autocomplete the first entry, which is the issue / MR author.

We broke this in 8.14 and fixed it in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7520, but it's broken again 😞