Fix flask mobile validate inventory (#583)

* [react] New # items in cart metrics + tags for existing metrics

* react fix

* [flask] fix broken checkout in mobile (no validate_inventory passed)

flask/src/main.py

@@ -108,7 +108,7 @@ def checkout():
     order = json.loads(request.data)
     cart = order["cart"]
     form = order["form"]
-    validate_inventory = order["validate_inventory"] == "true"
+    validate_inventory = "validate_inventory" in order and order["validate_inventory"] == "true"
 
     inventory = []
     try: