user upgrades: add UserUpgrade model.

Add a model to store the status of user upgrades.

* Store the upgrade purchaser and the upgrade receiver (these are
  different for a gifted upgrade, the same for a self upgrade).
* Store the upgrade type: gold, platinum, or gold-to-platinum upgrades.
* Store the upgrade status:
** pending: User is still on the Stripe checkout page, no payment
   received yet.
** processing: User has completed checkout, but the checkout status in
   Stripe is still 'unpaid'.
** complete: We've received notification from Stripe that the payment
   has gone through and the user has been upgraded.
* Store the Stripe checkout ID, to cross-reference the upgrade record on
  Danbooru with the checkout record on Stripe.

This is the upgrade flow:

* When the user clicks the upgrade button on the upgrade page, we call
  POST /user_upgrades and create a pending UserUpgrade.
* We redirect the user to the checkout page on Stripe.
* When the user completes checkout on Stripe, Stripe sends us a webhook
  notification at POST /webhooks/receive.
* When we receive the webhook, we check the payment status, and if it's
  paid we mark the UserUpgrade as complete and upgrade the user.
* After Stripe sees that we have successfully processed the webhook,
  they redirect the user to the /user_upgrades/:id page, where we show
  the user their upgrade receipt.

config/routes.rb

@@ -254,7 +254,7 @@ Rails.application.routes.draw do
       get :custom_style
     end
   end
-  resource :user_upgrade, :only => [:new, :create, :show]
+  resources :user_upgrades, only: [:new, :create, :show]
   resources :user_feedbacks, except: [:destroy]
   resources :user_name_change_requests, only: [:new, :create, :show, :index]
   resources :webhooks do