Straighten out reorder permissions on form configurations

Summary:
Fixes T10012. The permissions here are little weird: you need edit permission on the configurations, not the engines. I was checking edit permission on the engines only.

I should possibly make this a bit more consistent, the engine edit permission is just very convenient to use to enforce object create permission right now. I'll likely clean this up after T9789.

Test Plan:

• Tried to reorder forms as a less-privileged user, got proper policy errors.
• Reordered forms normally as a regular user.

Reviewers: chad

Reviewed By: chad

Subscribers: Luke081515.2

Maniphest Tasks: T10012

Differential Revision: https://secure.phabricator.com/D14824