Implement a "pro" EditEngine for dashboard panels

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10855

Differential Revision: https://secure.phabricator.com/D16226