Rooms notifications in the spaces tray

I think it would be very helpful to be able to see which of your spaces have open rooms, especially once we get more people into Realm and everyone’s spaces tray grows.

Start by locating the spaces tray and the code that represents open rooms, then review the attached screenshot for the intended presentation. Done should mean users can tell which spaces have open rooms from the tray, with the behavior and visual details agreed before implementation.

Written by the indexing model from the issue text.