Add a footer to PHUITwoColumnView

Summary: This allows setting of full-width content underneath the two column, or full column all by itself. Maybe these names are bad.

Test Plan: Using these in Differential / Diffusion conversions.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15455