Allow users to mark themselves as "Available", "Busy" or "Away" while attending an event

Summary:
Ref T11816.

• Now that we can do something meaningful with them, bring back the yellow dots for "busy".
• Default to "busy" when attending events (we could make this "busy" for short events and "away" for long events or something).
• Let users pick how to display their attending status on the event page.
• Also show which event the user is attending since I had to mess with the cache code anyway. We can get rid of this again if it doesn't feel good.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16802