REFACTOR: Use the Prices API in place of Plans (#17)

Stripe has a newer API called Prices where you can create a price for any product and it can either be recurring or one-time. The easy part is existing Plans work with the Prices API by passing a Plan ID, but objects are returned in the slightly-different Prices API object format.

This commit is a refactor to the new API to handle the data in its new form, and lays the foundation for a one time payment plan to be added to any subscriptions product.

test/javascripts/components/payment-options-test.js.es6

@@ -24,8 +24,16 @@ componentTest("Discourse Subscriptions payment options has content", {
 
   beforeEach() {
     this.set("plans", [
-      { currency: "aud", interval: "year", amountDollars: "44.99" },
-      { currency: "gdp", interval: "month", amountDollars: "9.99" }
+      {
+        currency: "aud",
+        recurring: { interval: "year" },
+        amountDollars: "44.99"
+      },
+      {
+        currency: "gdp",
+        recurring: { interval: "month" },
+        amountDollars: "9.99"
+      }
     ]);
 
     this.set("planTypeIsSelected", true);