[Redesign] Workboard headers / icons

Summary: Ref T8099, cleans up spacing, colors. Switched back to dropdown icon.

Test Plan: test headers in workpanels, conpherence

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13080