Add setHeader to PHUITwoColumnView for consistent page layouts

Summary: Working towards making PHUITwoColumnView into a page layout engine. Adds header support.

Test Plan: Use new header on Profile and Profiles. No visual changes, less duplicated code.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15292