FEATURE: Allow one-time purchases on products (#18)

Building off the foundation of using the Prices API, this PR adds the ability to create a one-time purchase plan for any product, which then can add a user to the specified plan group.

Some things to be aware of:

    One-time purchases cannot have trials.
    One-time purchases use the Invoice API instead of Subscriptions. Invoices are created then charged immediately.
    Users should receive emails for these invoices directly from Stripe just like subscriptions.

spec/requests/admin/plans_controller_spec.rb

@@ -98,7 +98,12 @@ module DiscourseSubscriptions
 
           it "creates a plan with an interval" do
             ::Stripe::Price.expects(:create).with(has_entry(recurring: { interval: 'week' }))
-            post "/s/admin/plans.json", params: { interval: 'week', metadata: { group_name: '' } }
+            post "/s/admin/plans.json", params: { type: 'recurring', interval: 'week', metadata: { group_name: '' } }
+          end
+
+          it "creates a plan as a one-time purchase" do
+            ::Stripe::Price.expects(:create).with(Not(has_key(:recurring)))
+            post "/s/admin/plans.json", params: { metadata: { group_name: '' } }
           end
 
           it "creates a plan with an amount" do