Option to modify the reschedule and cancelation redirrects

[Valentina R]

Useful when using a custom-built my-appointments page with the my-appointments custom shortcode (provided by us).

Requested here.

[Željan Topić]

Added 'woocommerce_appointments_cancel_appointment_redirect' & 'woocommerce_appointments_reschedule_appointment_redirect' to filter the redirects from 4.13.2.