#31 API POST Compensation

* adds initialize_objects to an abstractmethod of the super class to be implemented in subclasses
* differentiates error messages if intervention does not exist or is just not shared with the user

api/utils/serializer/v1/compensation.py

@@ -81,8 +81,10 @@ class CompensationAPISerializerV1(AbstractModelAPISerializerV1):
         """
         intervention = Intervention.objects.get(
             id=intervention_id,
-            users__in=[user],
         )
+        is_shared = intervention.is_shared_with(user)
+        if not is_shared:
+            raise PermissionError("Intervention not shared with user")
         obj.intervention = intervention
         return obj