Better CSS for mobile workboards

Summary: Fixes T8964 and a few other details. Cards float to 100%, headers too, policy moved to it's own line.

Test Plan: Mobile workboards at 320 and 360px wide.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8964

Differential Revision: https://secure.phabricator.com/D13722