Federated models

Sorry @Marjolaine another thing 😅

federated models in DjangolDP currently don't support NOT NULL fields (because external resources will be created as "backlinks" which have only a urlid field set), so all fields need to be made null=True and/or blank=True where applicable

All models which should be federated (should be able to have ForeignKey or ManyToManyField references with external resources) need to have rdf_type defined in the model Meta (corresponding to the RDF type they represent in the ontology) e.g.

rdf_type = 'http://energie-partagee-ontology/owl#xyz'

Do you have an ontology on this project ? If not you can technically just make this anything unique, DjangoLDP doesn't validate it 😅