Compensation users refactoring

* moves accessibility of compensations completely into related intervention.users set
  * this way the visibility for a compensation can not differ from the one of the 'parent' intervention

compensation/models.py

@@ -100,9 +100,6 @@ class Compensation(BaseObject):
         related_name='compensations'
     )
 
-    # Users having access on this object
-    users = models.ManyToManyField(User)
-
     @staticmethod
     def _generate_new_identifier() -> str:
         """ Generates a new identifier for the intervention object
@@ -153,3 +150,6 @@ class EcoAccount(Compensation):
     with some kind of currency. From this account one is able to 'withdraw' currency for current projects.
     """
     handler = models.CharField(max_length=500, null=True, blank=True, help_text="Who is responsible for handling the actions")
+
+    # Users having access on this object
+    users = models.ManyToManyField(User)