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.

app/controllers/discourse_subscriptions/subscriptions_controller.rb

@@ -27,12 +27,17 @@ module DiscourseSubscriptions
 
     def create
       begin
-        plan = ::Stripe::Plan.retrieve(params[:plan])
+        plan = ::Stripe::Price.retrieve(params[:plan])
+
+        if plan[:metadata] && plan[:metadata][:trial_period_days]
+          trial_days = plan[:metadata][:trial_period_days]
+        end
 
         @subscription = ::Stripe::Subscription.create(
           customer: params[:customer],
-          items: [ { plan: params[:plan] } ],
+          items: [ { price: params[:plan] } ],
           metadata: metadata_user,
+          trial_period_days: trial_days
         )
 
         group = plan_group(plan)