FEATURE: Add support for 3D Secure payments (#19)

Adds an additional checkout flow to support authentication of payment methods.

assets/javascripts/discourse/models/transaction.js.es6

@@ -0,0 +1,12 @@
+import { ajax } from "discourse/lib/ajax";
+
+export default {
+  finalize(transaction, plan) {
+    const data = {
+      transaction: transaction,
+      plan: plan
+    };
+
+    return ajax("/s/subscriptions/finalize", { method: "post", data });
+  }
+};