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.
Exercised all the group/sort options. Ran the index script over my 100k task
corpus. Edited task-project membership and verified the index updated.

Reviewed By: cadamo
Reviewers: gc3, jungejason, cadamo, tuomaspelkonen, aran
CC: aran, cadamo, epriestley
Differential Revision: 556