Add a "group by priority" to the homepage revision query

Summary: The effect of this is just to order tasks by (priority, modified)
instead of (modified), i.e. in the same default order as Maniphest, so the top
10 tasks here are the top 10 tasks in your assigned list.

Test Plan: Looked at "Assigned Tasks" on the homepage.

Reviewers: fratrik, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1621