Update EditEngine pages to take a page header separate

Summary: This simplifies EditEngine pages in general by removing the dual header, and extending to allow setting of a custom PHUIHeaderView if needed (like settings).

Test Plan:
Review all settings pages, review task, project pages. This should all be fine, but is a big change maybe some layouts I'm not considering. Tested these all mobile, destkop as well.

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Differential Revision: https://secure.phabricator.com/D18527