Merge branch 'restore-order-children' into 'master'

Restore order by ID on Element children list

Closes #602

See merge request !1150

arkindex/documents/api/elements.py

@@ -587,7 +587,7 @@ class ElementChildren(ElementsListMixin, DestroyModelMixin, ACLMixin, ListAPIVie
         return filters
 
     def get_order_by(self):
-        return ('paths__ordering', )
+        return ('paths__ordering', 'id')
 
     def get_queryset(self):
         role = Role.Guest if self.request.method in permissions.SAFE_METHODS else Role.Contributor