Add ability to mention a team

[marcoambrosini]

Teams can be added and removed from conversations. It would be nice if we could also mention them.

[nickvergessen]

https://github.com/nextcloud/server/blob/ec5133b739eabc76271789504b4dbb91a534f552/lib/private/Comments/Comment.php#L223-L222 was prepared recently, so this should be possible going forward.