Give Phriction documents modern string status constants instead of numeric constants

Summary:
Depends on D19099. Ref T13077. Updates Phriction documents to string constants to make API interactions cleaner and statuses more practical to extend.

This does not seem to require any transaction migrations because none of the Phriction transactions actually store status values: status is always a side effect of other edits.

Test Plan: Created, edited, deleted, moved documents. Saw appropriate UI cues. Browsed and filtered documents by status in the index.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19100