Phriction - consolidate edit business logic into Editor
Ref T4029. Some business logic lives outside the editor. This revision moves that logic from the edit controller into the editor proper. This makes re-using that business logic across other endpoints - say like a conduit end point - possible. This is also part of the general modernization quest for phriction I am on.
This diff also restores the functionality where you can delete a document by wiping out the content and saving.
Test Plan: tried to make a document with no title or content and saw errors. opened a document for edit with user 1, then made edits with user 2, then saw an error when i made the edit with user 1. clicking "overwrite changes" then worked. deleted a document by wiping out the body and clicking save.
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T4029
Differential Revision: https://secure.phabricator.com/D10795