Saturate day of month in datepicker

Summary:
The datepicker could step by the wrong number of months, due to the date rolling over to the next month when the number of days in the month is exceeded. For example, going forward from January 31 would jump to March 3, while going backward from July 31 would only go to July 1.

Push the date back to ensure that the datepicker stays in the correct month when switching.

Test Plan: Changed months starting from an assortment of dates.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: artms, Korvin

Differential Revision: https://secure.phabricator.com/D21673