Apront Two Column View

Summary: Wanted to pull this out in case we don't use it in Maniphest, still useful perhaps in the future. Creates a sidebar that wraps when on mobile.

Test Plan: Tested UIExample

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5321