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/application_controller.rb

@@ -136,6 +136,8 @@ class ApplicationController < ActionController::Base
       render_error_page(422, exception, template: "static/tag_limit_error", message: "You cannot search for more than #{CurrentUser.tag_query_limit} tags at a time.")
     when PostQuery::Error
       render_error_page(422, exception, message: exception.message)
+    when UpgradeCode::InvalidCodeError, UpgradeCode::RedeemedCodeError, UpgradeCode::AlreadyUpgradedError
+      render_error_page(422, exception, message: exception.message)
     when RateLimiter::RateLimitError
       render_error_page(429, exception, message: "Rate limit exceeded. You're doing that too fast")
     when PageRemovedError

app/controllers/upgrade_codes_controller.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class UpgradeCodesController < ApplicationController
+  respond_to :js, :html, :json, :xml
+
+  def index
+    @upgrade_codes = authorize UpgradeCode.visible(CurrentUser.user).paginated_search(params, count_pages: true)
+    respond_with(@upgrade_codes)
+  end
+
+  def redeem
+  end
+
+  def upgrade
+    @upgrade_code = UpgradeCode.redeem!(code: params.dig(:upgrade_code, :code), redeemer: CurrentUser.user)
+
+    respond_with(@upgrade_code, location: @upgrade_code.user_upgrade)
+  end
+end

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