Separate handle "status" and "availability"

Summary:
Ref T7707. Handles currently have a "status" field and a "disabled" field.

The "status" field has these possible values: "open", "closed", "1", "2". durp durp durp

Instead, do:

• status = <open, closed>
• availability = <full, partial, none, disabled>

I think these make more sense? And are a bit more general? And use the same kind of constants for all values!

Test Plan: Looked at all affected handles in all states (probably).

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7707

Differential Revision: https://secure.phabricator.com/D12832