Implement query and policy rules for subprojects

Summary:
Ref T10010. This implements technical groundwork for subprojects. Specifically, it implements policy rules like Phriction:

• to see a project, you must be able to see all of its parents (and the project itself).
• you can edit a project if you can edit any of its parents (or the project itself).

To facilitiate this, we load all project ancestors when querying projects so we can do the view/edit checks.

This does NOT yet implement:

• proper membership rules for these projects (up next);
• any kind of UI to let users create subprojects.

Test Plan:

• Added unit tests.
• Executed unit tests.
• Browsed Projects (no change in behavior is expected).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14861