Varied models in an `ldp:Container`
An LDPContainer does not have to contain all objects of one RDF type
When using RDF data (#369 ) there's no work to be done to support that model-side, it just works. But there will probably be work needed for the serializer
We discussed how it impacts the cache: #365 (comment 57407)
SQL-side it may be possible for a package to do it by using django-polymorphic
>>> Project.objects.all()
[ <Project: id 1, topic "Department Party">,
<ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">,
<ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]