Modularize Dashboard Panel transactionns

Summary: Depends on D20369. Ref T13272. Move toward a world where we can edit panels with just one controller, instead of separate "Edit" and "Editpro" controllers.

Test Plan: Created and edited panels. This will get vetted more thoroughly after additional changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20370