Allow editing shipment reference

This is needed when we need to recreate the shipping if the carrier APIs did
not work correctly.

issue10310
review345991002

stock.py

@@ -185,8 +185,6 @@ class ShippingMixin:
     @classmethod
     def __setup__(cls):
         super().__setup__()
-        # The shipment reference will be set by the shipping service
-        cls.reference.readonly = True
         cls._buttons.update({
                 'create_shipping': {
                     'invisible': (Eval('reference', False)