Added Voucher expiration check in form using model property.

django_lostplaces/lostplaces/forms.py

@@ -26,6 +26,10 @@ class ExplorerCreationForm(UserCreationForm):
             self.add_error('voucher', 'Invalid voucher')
             return False
 
+        if not submitted_voucher.valid:
+            self.add_error('voucher', 'Expired voucher')
+            return False
+
         fetched_voucher.delete()
         return True