Make queries for Project "X" mean "X, or any subproject of X"

Summary:
Ref T10010. I think this is the desired/expected default behavior (e.g., searching for "Maniphest" should find tasks in any subproject or sprint of that project).

I'll probably add an "exact(...)" function later to mean "only the Maniphest superproject, exactly, not any of its children".

Test Plan:

• Added and executed unit tests.
• Ran various queries from the web UI.
• Got sensible-seeming results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14910