Allow 'Backdating' of first pro rata payment on subscription to date of Trial

If a child attends a trial, our T&Cs require that if they sign up to a subscription after, then the subscription starts from the following week.

Some parents ‘game the system’ by delaying their subscription sign up because, say, their child has a party to go to the following week.

The system currently only pro ratas from the time of signing up: it doesn’t charge for those ‘missed’ classes. So we then have to manually charge them to backdate for the missed classes.

We would like the system to do this automatically.