Move Dashboards main edit flow to EditEngine

Summary:
Depends on D20402. Ref T13272. Replaces an old-school hard-coded "EditController" with a more modern one.

The actual panel stuff is still using a weird mix of legacy manual save() calls, but that's up next.

Test Plan: Created Dashboards, edited all dashboard fields via "Edit Dashboard".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20403