R1:fef3c778fd45

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…