(no-ticket) Change default post_url_continue value to None in some response_add admin methods

Before this change, post_url_continue argument passed to response_add
had the default value hardcoded as '../%s/' in some admin mixins.
This caused Django to calculate a valid url in previous versions by
substituting '%s'. Now Django only does this when the argument's value
is equal to None.

Change-Id: Idb2941f391ea202fe9c4f84084dc8732bb1d2974