Prebooking users #3

Add to the documentation a description of how to set up a one-time user accounts that have a pre-booked session on a particular piece of kit. Use case: group work in synchronous f2f class - if offer general link, a group might book one piece of kit each, leaving other groups with no kit they can book. If we pre-create a user for each group, and hand each group a link to the booking system that is unique to that user, then there is no more kit they can book with that account anyway, and each group would only get one link (could print and distribute, or email to group leader depending on how far in advance group membership is known)