Implement PHUIHeaderView from AphrontTagView

Summary: Looking at implementing ManiphestTaskListView as standard components, need to add this functionality for Headers.

Test Plan: Browsed various pages, couldn't spot any regressions offhand.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12802