Define `class_type` property in the model's inner Meta class

[edmondchuc]

This should probably be inside the inner Meta class instead of a direct property on the model.

class Concept(Common):
-  class_type = models.IRIField(predicate=RDF.type, value=SKOS.Concept)
   pref_label = models.CharField(predicate=SKOS.prefLabel, lang='en', required=True)
   alt_labels = models.CharField(predicate=SKOS.altLabel, lang='en', many=True)
   definition = models.CharField(predicate=SKOS.definition, lang='en', required=True)
   children = models.IRIField(predicate=SKOS.narrower, many=True, inverse=SKOS.broader)
   other_ids = models.CharField(predicate=SKOS.notation, many=True)
   home_vocab_uri = models.IRIField(predicate=RDFS.isDefinedBy)

+ class Meta:
+     class_type = [SKOS.Concept]

   def __repr__(self):
       return f'<{self.__uri__}>'