Clean up some EditEngine meta-policies

Summary:
Ref T9908. Simplify some of the policies here:

• If you can edit an application (currently, always "Administrators"), you can view and edit all of its forms.
• You must be able to edit an application to create new forms.
• Improve some error messages.
• Get about halfway through letting users reorder forms in the "Create" menu if they want to sort by something weird since it'll need schema changes and I can do them all in one go here.

Test Plan:

• Tried to create and edit forms as an unprivileged user.
• Created and edited forms as an administrator.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14700