Future Booking Capabilities

Allow members to book classes in advance even when they dont have the credits available, only in the following circumstances:

1. When they buy a membership that starts in the future, allow them to make bookings from the start date onwards

2. When they are a member but they run out of credits, allow them to book into the next month (when they will have credits again if they pay)

This will involve having a credit account balance that can go into negative and then gets topped back up when they pay or the membership starts.