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