Build an AphrontMultiColumnView, use on workboards.

Summary: Abstract out the multi-column code from workboards and have it be available separately. I feel like there will be some benefit here especially for custom developers in how they present infromation (like releeph). It also scales back to tablet and mobile fairly well, so they get those things for free.

Test Plan: Tested mobile, tablet and chrome layouts.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5529