Created|Deleted refactoring

* refactors base attributes created and deleted into UserActionLogEntry foreign keys
* refactors all related queries and process logic
* fixes binding_on into binding_date in intervention/detail/view.html
* adds basic __str__ for some models
*

intervention/tables.py

@@ -50,7 +50,7 @@ class InterventionTable(BaseTable):
     lm = tables.Column(
         verbose_name=_("Last edit"),
         orderable=True,
-        accessor="created_on",
+        accessor="created__timestamp",
     )
     """
     # ToDo: Decide to keep actions column or to dismiss them