user upgrades: add links to Stripe payment & receipt page.

Add links to the Stripe payment page and the Stripe receipt page on
completed user upgrades.

The Stripe payment link is a link to the payment details on the Stripe
dashboard and is only visible to the owner.

app/controllers/user_upgrades_controller.rb

@@ -27,6 +27,16 @@ class UserUpgradesController < ApplicationController
     respond_with(@user_upgrade)
   end
 
+  def receipt
+    @user_upgrade = authorize UserUpgrade.find(params[:id])
+    redirect_to @user_upgrade.receipt_url
+  end
+
+  def payment
+    @user_upgrade = authorize UserUpgrade.find(params[:id])
+    redirect_to @user_upgrade.payment_url
+  end
+
   private
 
   def recipient