Add a 7th column to AphrontMultiColumnView

Summary: Adds a 7th column, useful for fluid calendars.

Test Plan: UIExamples

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2898

Differential Revision: https://secure.phabricator.com/D5597