Add "Spaces", an application for managing policy namespaces

Summary: Ref T3820. This doesn't actually do anything yet, but dumps in all the plumbing.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, wienczny, jdloft, devurandom, thz, hwinkel, 20after4, sascha-egerer, seporaitis, joshuaspence, chad, epriestley

Maniphest Tasks: T3820

Differential Revision: https://secure.phabricator.com/D9204