PHUIDocument view for mobile devices.

Summary: Adds collapsing of the sidebar, also allows you to say where it goes on mobile (above or below content). ToC for example, above. General Navbar, below. Up to you.

Test Plan: Review UIExamples and Diviner.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6150