Improve Space behavior for subprojects and milestones

Summary:
Depends on D19549. Ref T13164. See PHI774.

• Make milestones inherit their parent project's space automatically, like they inherit their parent policies.
• Make subprojects default to their parent project's space.

Test Plan: Created subprojects and milestones, got sensible default/effective Space behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19550