Allow board columns to be reordered

Summary: Fixes T4567. This isn't going to win design awards and we have some leaky CSS, but it works fine.

Test Plan: {F176743}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4567

Differential Revision: https://secure.phabricator.com/D9905