Add Shadow Panels to PhabricatorMultiColumnView

Summary: This adds the ability to have a multi-column full height container that is responsive based on PHUIBox's shadow box.

Test Plan: Tested new examples in UIExamples and Workboards.

Reviewers: epriestley, edward, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5996