Allow users to hide rooms in grid schedule

[rixx]

The pretalx schedule editor does this – when rooms are hidden, there's a fixed-position menu at the lower edge of the screen that shows the list of hidden rooms / allows you to un-hide rooms.

Motivation: Events often have a lot of rooms, and some may not be relevant to some (or most) attendees, like workshop rooms, rooms only in use at the end of the event etc. Hiding those rooms (and persisting the hiding via localStorage) would go a long way in making the schedule easier to navigate.