- only within current school term
- no duplicates
- unique ID for every element

id -> pk
datetime.now needs timezone
get_or_create instead of just create -> could be called several times
ParticipationStatus is a CalendarEvent -> more args

This avoids a name collision with Kolego.

& use in DocumentationBatchCreateOrUpdateMutation & AbsencesBatchCreateMutation.

& share between query & mutation.