Added response parameter to postMessage data

no issue

- The 'response' property is needed to make the receiving client generic in regards of what data format has to be returned as a result of the call. For example with this change the 'response: subscription' meant there should be an incoming 'message' event as a response to this call with 'subscription' key into it's data

ghost/admin/app/components/gh-billing-update-button.js

@@ -33,7 +33,8 @@ export default Component.extend({
                 if (!fetchingSubscription && !this.get('subscription')) {
                     fetchingSubscription = true;
                     iframe.contentWindow.postMessage({
-                        query: 'getSubscription'
+                        query: 'getSubscription',
+                        response: 'subscription'
                     }, '*');
                 }
             }