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.
• Viewed Conpherence calendar.
• Load Conduit console.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8183

Differential Revision: https://secure.phabricator.com/D12840