Partially clean up Phriction document status constants; introduce "phriction.document.search"

Summary:
Depends on D19098. Ref T13077.

Phriction status constants currently use the "bag of statuses" approach typical of older code, and store integers in the database.

This fixes the "bag of statuses" stuff; a future change will fix the integers.

Also adds a skeleton for phriction.document.search, but doesn't implement the Conduit interface yet.

Test Plan: Searched for documents with various status constraints. Grepped for removed status constants. Viewed document list.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19099