feat: Ensure shipping is locked if cart is frozen (viur-framework#131)

Author: sveneberth

src/viur/shop/skeletons/cart.py

@@ -152,8 +152,10 @@ def unserialize_compute(self, skel: "SkeletonInstance", name: str) -> bool:
             # FIXME: Ensure it's a valid shipping for the cart
             return False
 
+        if skel["is_frozen"]:  # locked, unserialize the latest stored value from entity
+            return False
+
         if skel["shipping_status"] == ShippingStatus.CHEAPEST:  # compute cheapest
-            # TODO: if not locked ...
             self._prevent_compute = True
             try:
                 applicable_shippings = SHOP_INSTANCE.get().shipping.get_shipping_skels_for_cart(