add missing date_str property

tabellio/documents/question.py

@@ -137,6 +137,12 @@ class Question(Item, BasePublication):
         else:
             return '%s (%s)' % (self.questype_str, self.session)
 
+    @property
+    def date_str(self):
+        if not self.date:
+            return ''
+        return self.date.strftime('%d/%m/%Y')
+
     @property
     def associated_docs(self):
         docs = []