Show autocompletion as soon as "@" is typed

In order to show the autocompletion it was needed to type at least
another character after "@", so only the mentions that matched that
character were shown. Now the autocompletion is shown as soon as "@" is
typed, which shows all the possible mentions in the room.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>