R1:de0c89261e9e

Allow Maniphest tasks to be filtered by Project

Summary:
Major things taking place here:

- A new table for storing <task, project> relationships.
- Moved all task query logic into a dedicated class.
- Added a "projects" filter to the UI.

I was originally going to try to drive this off the main search index but the
perf benefits of a custom schema make an overwhelming argument in favor of doing
it this way.

Test Plan:
Filtered tasks by author and owner and zero, one, and more than one project.