Implement a basic project.search third-generation API method

Summary: Ref T10010. This still needs support for attachments (to get members) and more constraints (like slugs), but mostly works.

Test Plan: Ran query, saw basically sensible results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14889