Issue #3559 - Make first day of the week configurable by locale

Addresses issue #3559 . 

1. Added a date_picker:first_day entry in the default locale file, with a value of 0
2. Made this value available to the JS, defaulting to 0 if not present, via the admin/shared/_translations.html.erb view template
3. Updated the datepicker invocation to pass this value to the datepicker when invoked

Developers can set the calendar to start on an alternate day by setting the date_picker:first_day entry to the appropriate number (0 for Sunday, 1 for Monday, etc.) in their locale file.