Implement "Group by Owner" on Workboards

Summary: Depends on D20269. Ref T10333. Now that orderings are modularized, this is fairly easy to implement. This isn't super fancy for now (e.g., no profile images) but I'll touch it up in a general polish followup.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10333

Differential Revision: https://secure.phabricator.com/D20270