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/instances_controller.rb

@@ -3,10 +3,12 @@
 module Admin
   class InstancesController < BaseController
     def index
+      authorize :instance, :index?
       @instances = ordered_instances
     end
 
     def resubscribe
+      authorize :instance, :resubscribe?
       params.require(:by_domain)
       Pubsubhubbub::SubscribeWorker.push_bulk(subscribeable_accounts.pluck(:id))
       redirect_to admin_instances_path