R1:afed8bb92955

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…