Add https://openactive.io/AttendeeAbsent for case where attendee did not attend #199

There is currently no way to distinguish between "customer did not attend", "opportunity attendance updates not supported" and "attendance data was not recorded".

This is resolvable by simply adding https://openactive.io/CustomerAbsent alongside https://openactive.io/CustomerAttended, which allows the booking system to confirm in either case.

And actually renaming these to https://openactive.io/AttendeeAbsent and https://openactive.io/AttendeeAttended to use correct terminology.