Add "Group by: Project" to Maniphest

Summary:
Allow tasks to be grouped by project. Since this is many-to-many and we're a little deficient on indexes for doing this on the database, we pull all matching tasks and group them in PHP. This shouldn't be a huge issue for any existing installs, though, and we can add keys when we run into one.

• When a task is in multiple projects, it appears under multiple headers.
• When a query has a task filter, those projects are omitted from the grouping (they'd always show everything, which isn't useful). Notably, if you search for "Differential", you can now see "Bugs", "Feature Requests", etc.

Test Plan: Selected "Group by: Project".

Reviewers: btrahan, Josereyes

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T923

Differential Revision: https://secure.phabricator.com/D1953