Add Spaces to Projects

Summary:
See PHI774. Ref T13164. There is no reason projects don't support Spaces, just a vague concern that it's not hugely useful and might be a bit confusing.

However, it's at least somewhat useful (to improve consistency and reduce special casing) and doesn't necessarily seem more confusing than Projects are anyway. Support is trivial from a technical point of view, so just hook it up.

Test Plan: Created new projects, shifted projects between spaces. The support is all pretty much automatic.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19549