Show waiting list option for every session in a drop-in block#92

Currently the waiting list is only shown to bookers when every session in the block is fully booked. We want to run a waiting list for each individual session. We also want to use drop-in tickets to allow bookers to checkout for multiple sessions in one go.

Yes. This is common sense, but I only realised the waiting list wasn’t on when multiple people parents came back to me saying there was no waiting list option on the fully booked sessions.