Remove signup

Fully removes the notion of signing up from the system, and replaces it with an event-global "open" and "closed" mode that manages when sessions are joinable. When sessions are closed, the buttons are dimmed out and disabled. When open, all sessions are joinable. 

Stripped out the "attend", "unattend", "start", and "stop" messages from throughout. Also pulled out various other UI pieces associated with that from the admin UI, plus the "your pick" tags on sessions.

This should be a far more simple system, and gets rid of the two stage process with sessions.