This tests mainly cover cases whose API user is friendly.
Should be extended with illegal things.
ActivityTag, ActivityTemplate and Place models got tests for root and
specific-event instance.
Event-based urls
- Add viewset mixin 'EventUrlViewSetMixin' to get the event from the
'event_pk' url kwarg of a view.
- Add url serializer fields for object which can be accessed with a
root-level and/or event-specific url ('EventHyperlinked*Field).
Update viewsets and serializers to use these + clean inheritance
viewsets.
