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"> ]