Add a way to move a single date/session in a block booking to another class without affecting any other dates

At the moment, the Move function in Book That In is restricted to moving the Whole Booking.

Sometimes, a customer will be allowed to move just a single session in their block booking to another class, as they cannot make their usual class.

It is not possible currently to move just the customer for just that one class - it is a two step process of updating the current booking to remove the session that won’t be attended, and then adding the booking for the single session manually to the new class.

It would be much handier to be able to simply move a single session.