On workboards, put older milestone columns on the right

Summary:
Ref T10349. Instead of showing columns in "Backlog, Custom, Sprint 1, Sprint 2, Sprint 3" order, show the sprints in reverse order: 3, 2, 1.

This makes it easier to get to the new stuff, and you don't have to drag over older stuff or archive it immediately.

Trello's own meta-board for Trello development is a good example of this in the wild: older stuff goes out to the right, so you can get to the newer stuff easily:

https://trello.com/b/nC8QJJoZ/trello-development

Test Plan: Saw board in 3, 2, 1 order instead of 1, 2, 3.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10349

Differential Revision: https://secure.phabricator.com/D15267