Api: Multi User Bookings

[daleighan]

This may be a feature for the future, but I would like to consider how it may be done at this point. There is now a web interface that could be easily adapted for this.

[daleighan]

I will probably do this by using an intermediary table called booking_users matching a booking to multiple users. There will also need to be a flag on the booking indicating whether the booking is single or multi-user.