Add Fluid-ish view to AphrontMultiColumnView

Summary: Cleans up some CSS while adding lots of other... Mainly, this allow min-width "tables" that trigger a scroll-bar, but go full width if larger than min.

Test Plan: Tested Workboard Examples and some Project pages, Chrome, Tablet and Mobile Layouts

Reviewers: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7509