Replace user "status" with "availability"

Summary:
Ref T7707. Ref T8183.

• Currently, user status is derived by looking at events they created. Instead, look at non-cancelled invites they are attending.
• Prepare for on-user caching.
• Mostly remove "Sporradic" as a status, although I left room for adding more information later.

Test Plan:

• Called user.query.
• Viewed profile.
• Viewed hovercard.
• Used mentions.
• Saw status immediately update when attending/leaving/cancelling a current event.
• Created an event ending at 6 PM and an event from 6:10PM - 7PM, saw "Away until 7PM".

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8183, T7707

Differential Revision: https://secure.phabricator.com/D12833