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/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