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.

app/controllers/webhooks_controller.rb

@@ -11,6 +11,9 @@ class WebhooksController < ApplicationController
     when "stripe"
       PaymentTransaction::Stripe.receive_webhook(request)
       head 200
+    when "shopify"
+      PaymentTransaction::Shopify.receive_webhook(request)
+      head 200
     when "discord"
       json = DiscordSlashCommand.receive_webhook(request)
       render json: json