Appointments+: use WP start day and date/time settings in backend

Hi!

The booking calendar (front-end) does follow the WordPress start day and date/time settings. That doesn't apply only to the back-end (configuration) settings so, for example, in Working Hours settings the first day of the week is always Sunday, regardless of WP settings.

It would be great if Appointments+ would follow WP settings in that matter in back-end as well. (original thread: https://premium.wpmudev.org/forums/topic/extra-settings-for-appointments#post-1270847).