Intervention model refactoring

* adds UserActionLogEntry model to user/models.py
  * wraps user and timestamp info
  * can be extended for more information in the future
* refactors all filtering and accessing on values

intervention/filters.py

@@ -117,7 +117,7 @@ class InterventionTableFilter(django_filters.FilterSet):
         """
         if not value:
             return queryset.filter(
-                recorded_on=None,
+                recorded=None,
             )
         else:
             return queryset