Migrate revision storage to modern status constants ("accepted") instead of legacy numeric values ("2")

Summary:
Ref T2543. Rewrites all the storage to use constants.

Note that transactions still use legacy values, I'll migrate and update them separately.

Test Plan:

• Ran migration.
• Browsed around, changed revision states, viewed dashboard, etc.
• Selected DISTINCT() and GROUP_CONCAT() of the status field in the database, saw sane/expected before and after values.
• Verified that old Conduit methods still return numeric constants for compatibility.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18418