RegistrationService lets people sign themselves up for a contest instead of waiting for an organiser to
assign them.
A contest can ask its registrants for details about themselves, expressed as the same attributes the space keeps on its members, and this service both publishes that form and takes the answers back. It is called by the contest client; there is no organiser-facing screen behind it, and organisers add people through ParticipantService instead. Registering is not the same as being admitted: where a contest requires admission, a registrant still has to be admitted through AdmissionService.