Allow for umpires to be assigned to bookings

Umpires will often be called on to officiate during a casual booking, but we currently have no way of tracking how many such games they have officiated, so we can't report on that in order to pay the referees.  It would be good to be able to allocated officials to bookings in the same way as we can for fixtures, and then include those official stats in reports so we can pay them correctly.