NotificationCard: Mentions refer to the source
NotificationCard: Mentions refer to the source

Instead of saying "mentioned you", say "mentioned @source".

Together with the previous few commits, this fixes #40, and fixes #44.

Signed-off-by: Gergely Nagy <0efc652ff6792fef6190e3f066133e37a050e52a@gergo.csillger.hu>

algernon/mad-tooter,algernon/mad-tooter