Support typeaheads in thread message input
The main message input supports the following typeaheads:
| TypeAhead name | screenshot |
|---|---|
| Command |  |
| User |  |
| Issue |  |
| Emoji |  |
The implementation is in the public/js/views/chat/typeaheads folder https://gitlab.com/gitlab-org/gitter/webapp/tree/develop/public/js/views/chat/typeaheads