First pass at new Workboard UI

Summary: Cleans up Workboards to match the mocks. No new functionality, just more consistent colors/spacing/common components.

Test Plan:
Visit a few workboards, drag and drop items. Mobile, Tablet, Desktop

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15115