Add whitelist mode (#11291)

app/controllers/about_controller.rb

@@ -3,6 +3,7 @@
 class AboutController < ApplicationController
   layout 'public'
 
+  before_action :require_open_federation!, only: [:show, :more]
   before_action :set_body_classes, only: :show
   before_action :set_instance_presenter
   before_action :set_expires_in
@@ -19,6 +20,10 @@ class AboutController < ApplicationController
 
   private
 
+  def require_open_federation!
+    not_found if whitelist_mode?
+  end
+
   def new_user
     User.new.tap do |user|
       user.build_account