Put "Push Policy" last in Diffusion, make editing Spaces work

Summary:
Ref T10923.

• The "Policy" edit form currently goes "Push, View, Edit". Reorder the defaults to "View, Edit, Push".
• Editing Spaces doesn't currently work: the element appears in the UI, but isn't actually processed when handling transactions. Make that work.

Test Plan:

• Edited a repository policies, saw "View, Edit, Push".
• Moved a repository between Spaces.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15866