Pulp

This may be up for discussion:
In the current state, the content serializer family can validate content objects containing artifacts, but when valid, the user is supposed to pop off the `_artifact{,s}` field and create the `ContentArtifact` entities on his own.
I think, if the serializer said, an object is valid, a subsequent `save` without further parameters should just work.

In my approach, i tried to follow the example in [0]. This has the nice side-effect, that the plugin writer is not anymore concerned with handling the `ContentArtifacts` at all.

[0] https://www.django-rest-framework.org/api-guide/serializers/#writing-create-methods-for-nested-representations