If a username is mentioned in a Conpherence thread/room that a user can see -- but is not a member of -- we should send them a notification:
> epriestley mentioned your username in Funny Cat Pictures.
If it fits reasonably in the UI, it would be nice to include a snippet of chat text in the notification, but this might not be very reasonable.
This allows users to be summoned to threads that they aren't necessarily following.