Add Spaces support to Phriction

Summary:
Ref T13164. See PHI774. Fixes T12435.

Since Phriction is hierarchical, there isn't a super strong motivation to support Spaces: you can generally set policies on a small number of documents to get the desired effective policy behavior.

However, it still improves consistency and there's no reason not to support Spaces. In the case where you have some moderately weird/complex policy on one or more Spaces, using Spaces to define the policy behavior can make things a bit simpler and easier to understand.

This probably doesn't actually fix whatever the root problem in T12435 was (complicated, non-hierarchical access policies?). See also a bunch of discussion in T12442. So we might end up going beyond this to address other use cases, but I think this is reasonable regardless.

Test Plan: Created and edited Phriction documents and shifted them between Spaces. Searched by Space, etc.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13164, T12435

Differential Revision: https://secure.phabricator.com/D19553