Add ownerheads to workboard cards

Summary: Reworks cards to add an assignee head and tooltip on workboards. This feels like a reasonable starting point, but they may move depending on feedback.

Test Plan:
View a lot of boards. Assign and unassign a task.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Luke081515.2, Korvin

Differential Revision: https://secure.phabricator.com/D15158