Convert Drydock lease and resource constants to strings

Summary:
Ref T9252. Drydock currently uses integer statuses, but there's no reason for this (they don't need to be ordered) and it makes debugging them, working with them, future APIs, etc., more cumbersome.

Switch to string instead.

Also rename STATUS_OPEN to STATUS_ACTIVE and STATUS_CLOSED to STATUS_RELEASED for consistency. This makes resources and leases have more similar states, and gives resource states more accurate names.

Test Plan: Browsed web UI, grepped for changed constants, applied patch, inspected database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14153