Fix start in fancy datepicker calendar

Summary:
Fixes T9675.
Fixes the calculation for the difference between the first day of the month and the first day of the week. It was previously possible for this to be negative, which caused the subsequent loop to hang the browser.

Test Plan:

• Set week to start on Sunday. Observed that the calendar still renders fine.
• Set week to start on Monday. Observed that the browser no longer hangs on November 2015.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T9675

Differential Revision: https://secure.phabricator.com/D14409