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