[Redesign] Remove PHUIActionHeaderView

Summary: This adds minor functionality to PHUIHeaderView (icons, tags) that were previously on in ActionHeader. This allows us to remove the PHUIActionHeaderView outright in favor of just one Header class.

Test Plan: Tested each callsite, workboards, hovercards, conpherence.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12902