Add moderator role and add pundit policies for admin actions (#5635)

* Add moderator role and add pundit policies for admin actions

* Add rake task for turning user into mod and revoking it again

* Fix handling of unauthorized exception

* Deliver new report e-mails to staff, not just admins

* Add promote/demote to admin UI, hide some actions conditionally

* Fix unused i18n

app/controllers/admin/subscriptions_controller.rb

@@ -3,6 +3,7 @@
 module Admin
   class SubscriptionsController < BaseController
     def index
+      authorize :subscription, :index?
       @subscriptions = ordered_subscriptions.page(requested_page)
     end