When scrolling forward a month in calendar date picker from 1/31, next chosen date should be 2/29, not 3/1.

Summary: Fixes T9295

Test Plan: Create event, open datepicker for start date, choose 1/31/2016, open datepicker again, click right button to scroll month. New suggested date should be 2/29/2016

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T9295

Differential Revision: https://secure.phabricator.com/D15727