[entities] Make ICKANPublishable adapter abstract

Better rely on explicit instanciation and selection in client code.

entities.py

@@ -42,6 +42,7 @@ def slugify(value):
 class CKANPublishableAdapter(EntityAdapter):
     """Adapter for entity that can be mapped to a CKAN dataset"""
     __regid__ = 'ICKANPublishable'
+    __abstract__ = True
     __select__ = (EntityAdapter.__select__ &
                   relation_possible('ckan_dataset_id', role='subject'))