Give workboard column header actions a more clickable appearance

Summary: Ref T13269. Make it visually more clear that the "Trigger" and "New Task / Edit / Bulk" dropdown menu items are buttons, not status icons or indicators of some kind.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13269

Differential Revision: https://secure.phabricator.com/D20332