Don't filter single retrieved event (detail view.)
Kulke: only set price when not free.
Add field which separates virtual super events.

Virtual super events are not present in the original
data but are generated to link recurring events together.
Add debugging related options to event serializer.
Don't create super events when only one child in db.
Remove redundant serializer.
Display recurring super events for now.
Kulke event name utility function.
Fix various issues in kulke super events.

Build names more lazily.
Super event keywords are an intersection of sub events' keywords.
Simplify date limit logic.
Hide internal field.