Spaces + Diffusion - make creating a new repository work again

Summary: Fixes T8702. Needed to teach these controls how to serialize / unserialize themselves in that they are storing two values (policy phid + space phid) nowadays

Test Plan: made a new repo successfully with policy as described in T8702. Also edited policy successfully.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8702

Differential Revision: https://secure.phabricator.com/D13472