Phriction - move "move" business logic to the editor

Summary: Ref T4029. Even more code consolidation and cleanup for the long term benefits!

Test Plan: moved a page successfully. tried to move a page to an existing page and got an error. did the two tab trick to try to move a deleted page and got an error.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4029

Differential Revision: https://secure.phabricator.com/D10812