Add users to file rooms on mentions
Add users to file rooms on mentions

Users are added on mentions in file-rooms,
so they can see the room in their room list and
the notification can be parsed and links to an existing room,
where they are a participant of.

Signed-off-by: Joas Schilling <ab43a7c9cb5b2380afc4ddf8b3e2583169b39a02@schilljs.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed