4873: Add "mention user" feature in review comments


What version are you running?


Describe the enhancement and the motivation for it.

Sometimes one needs input from a user who is not in the participants list. Something similar to @user.name on GitHub/GitLab/etc which can be inserted in a comment and once published the corresponding user will receive email notification with a link to the above comment. Certainly, autocompletion after @ is inserted would be useful.