Popup datepicker should respect user preferred week start day

Summary: Fixes T8605, Popup datepicker should respect user preferred week start day

Test Plan: Edit event, open datepicker, calendar weeks should start on Sunday by default, and another day, if specified in User Preferences.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8605

Differential Revision: https://secure.phabricator.com/D13419