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:
{F1904179}

{F1904180}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

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

resources/sql/autopatches/20161104.calendar.01.availability.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_calendar.calendar_eventinvitee
+  ADD availability VARCHAR(64) NOT NULL;

resources/sql/autopatches/20161104.calendar.02.availdefault.sql

@@ -0,0 +1,3 @@
+UPDATE {$NAMESPACE}_calendar.calendar_eventinvitee
+  SET availability = 'default'
+  WHERE availability = '';