Included cancellation_reason in subscription serialization (#12411)

refs #12403 

This new property needed to be exposed in serialization so that Ghost-Admin
can use it via the API.

core/server/models/stripe-customer-subscription.js

@@ -39,6 +39,7 @@ const StripeCustomerSubscription = ghostBookshelf.Model.extend({
             start_date: defaultSerializedObject.start_date,
             default_payment_card_last4: defaultSerializedObject.default_payment_card_last4,
             cancel_at_period_end: defaultSerializedObject.cancel_at_period_end,
+            cancellation_reason: defaultSerializedObject.cancellation_reason,
             current_period_end: defaultSerializedObject.current_period_end
         };
     }