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

resources/sql/patches/051.projectfilter.sql

@@ -0,0 +1,6 @@
+CREATE TABLE phabricator_maniphest.maniphest_taskproject (
+  taskPHID varchar(64) BINARY NOT NULL,
+  projectPHID varchar(64) BINARY NOT NULL,
+  PRIMARY KEY (projectPHID, taskPHID),
+  UNIQUE KEY (taskPHID, projectPHID)
+);