Make Workboard and Workpanel views extend AphrontTagView

Summary: Ref T1344. I need to put sigils on these for drag-and-drop.

Test Plan: Renders the same. Put sigils on 'em.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7940