BookingWP - WooCommerce Appointments 4.21.0

* Improvement - Send customer email notification when unpaid appointments are canceled.
* Improvement - Updated Product Add-ons to 4.1.0.
* Improvement - Percentage based fee for add-ons for NON Appointable products.
* Improvement - Better sanitization for the appointment ID display.
* Improvement - Removed "WooCommerce Create Customer on Order" integration.
* Improvement - Updated all translations.
* Fix - Reschedule button does not work outside My account pages.

* Feature - Phone orders: add customer (billing) data, when adding appointments in admin.
* Improvement - Compatibility with WooCommerce 5.4.
* Improvement - Compatibility with WordPress 5.8.
* Improvement - Updated all translations.
* Improvement - Option to skip cache for the "get_appointment_ids_from_order_item_id" method.
* Improvement - Updated Product Add-ons to 4.0.0.
* Fix - Add-ons console error when required fields are selected.
* Fix - Availability filter widget not counting for the start day.
* Fix - Missing closing <div> on the appointment edit screen.
* Fix - Select add-ons do not show selected options on page reload.
* Fix - When using radio select add-ons, the providers selection is reset on page reload.
* Fix - Cart adding appointments twice.
* Fix - Staff all together not working ok.
* Fix - Selecting staff can result in rendering HTML instead of staff images.

* Improvement - Compatibility with WooCommerce 5.2.
* Improvement - Updated Product Add-ons to 3.7.0.
* Improvement - Updated all translations.
* Improvement - Only add appointment status in 'view-order' endpoint, order already has this.
* Improvement - REST-API filter appointments by customer_id.
* Improvement - Availability widget filter should include the end date.
* Improvement - Added 'wc_appointments_before_find_scheduled_day_slots' for filtering posted data.
* Fix - Default date not working correctly.
* Fix - Auto updater not working.
* Fix - Staff selection now only loads slots for the selected staff.
* Fix - Cancel and Reschedule buttons now have nowrap style to prevent letter breaking.