Waitlisting for services!

We definitely need to control how many people are booked into a service.

It would be ideal if a waitlist option could be added to services. Then if a session is full, potential attendees can put themselves on the waitlist. If the team decides enough staff are free to facilitate a larger session, then they can add people from the waitlist into the session. 

Currently relying on customers to reach out if they wanted a particular session that was full, however, they a) can't even see that a session is there but full (calendar just shows blank); and b) we don't necessarily want to advertise that you can contact to find out about full sessions as then the team would likely have many emails asking for training outside of advertised times....