Implement a "pro" EditEngine for dashboard panels
Ref T10855. This can't replace the old edit flow yet, but get the basics in place.
(This is actually much closer to just being able to swap than I anticipated since CustomFields sort of just work, but the exiting flow has some "clone existing panel" / "place directly on dashboard" stuff that this doesn't yet.)
Test Plan: Created and edited a panel by manually using the "editpro" flow.
Reviewed By: chad
Maniphest Tasks: T10855
Differential Revision: https://secure.phabricator.com/D16226