Add back maniphest group headers to Dashboards
Summary: Fixes T6364. We should eventually remove the custom CSS from Maniphest, for now just share the styles.
Test Plan:
Add a dashboard panel to my home dashboard with GROUP BY set to Assigned on Maniphest Tasks.
Reviewers: epriestley, btrahan
Reviewed By: btrahan
Subscribers: hashar, Korvin, epriestley
Maniphest Tasks: T6364
Differential Revision: https://secure.phabricator.com/D10733