R1:2bc9dec85f03

Remove "status" field from events

Summary:
Ref T8183. See that task for discussion.

- For now, events always mark users as "Away".
- In the future, we may reintroduce "sporradic" or other more complicated availability states, but they would be properties of the invitee, not of the event itself.
- This also removes the long-deprecated `user.addstatus` and `user.removestatus` Conduit calls.

Test Plan:
- Created, edited, viewed events.
- Grepped for removed symbols.
- Viewed profile calendar.