Include number of affected people in all/group mention autocomplete item.
As mentioned in #2054.
To minimize misuse of all/group mentions, the autocomplete title includes the number of members this mention will notify, so users will be more considerate what groups they mention.
Example:
at-all "**all** All Project and Group Members (12)"
at-gitlab "**gitlab** GitLab (12)"
See merge request !1596