misc: always define cell.ajax_refresh (#29152)

combo/data/models.py

@@ -506,6 +506,9 @@ class CellBase(six.with_metaclass(CellMeta, models.Model)):
     # Apply changes to the template context that must visible to all cells in the page
     modify_global_context = None
 
+    # if set, automatically refresh cell every n seconds
+    ajax_refresh = None
+
     class Meta:
         abstract = True