Phriction - consolidate edit business logic into Editor

Summary:
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.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4029

Differential Revision: https://secure.phabricator.com/D10795