Add outbox attribute to instance actor (#14721)

It's not useful for now, but it's required by ActivityPub

app/controllers/instance_actors_controller.rb

@@ -17,6 +17,6 @@ class InstanceActorsController < ApplicationController
   end
 
   def restrict_fields_to
-    %i(id type preferred_username inbox public_key endpoints url manually_approves_followers)
+    %i(id type preferred_username inbox outbox public_key endpoints url manually_approves_followers)
   end
 end