Group Maniphest Tasks by Priority on Profiles

Summary: Ref T12423. Set the grouping by priority. Note this doesn't render headers but I don't want to spend a lot of time on this.

Test Plan: Review tasks in my sandbox, see them ordered by priority.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12423

Differential Revision: https://secure.phabricator.com/D18046