user upgrades: add upgrade code system.

Add a system for upgrading accounts using upgrade codes. Users purchase
an upgrade code off-site then redeem it on-site to upgrade their account
to Gold. Upgrade codes are randomly pre-generated and are one time use
only. Codes have enough randomness that guessing a code is infeasible.

config/danbooru_default_config.rb

@@ -392,6 +392,14 @@ module Danbooru
       true
     end
 
+    # The URL of the Shopify checkout page where account upgrades are sold.
+    def shopify_checkout_url
+    end
+
+    # The secret used to verify webhooks from Shopify. Get it from the https://xxx.myshopify.com/admin/settings/notifications page.
+    def shopify_webhook_secret
+    end
+
     def stripe_secret_key
     end
 

config/routes.rb

@@ -248,6 +248,13 @@ Rails.application.routes.draw do
   resources :tags
   resources :tag_aliases, only: [:show, :index, :destroy]
   resources :tag_implications, only: [:show, :index, :destroy]
+
+  get "/redeem", to: "upgrade_codes#redeem", as: "redeem_upgrade_codes"
+  resources :upgrade_codes, only: [:create, :index] do
+    get :redeem, on: :collection
+    post :upgrade, on: :collection
+  end
+
   resources :uploads do
     collection do
       get :batch, to: redirect(path: "/uploads/new")