admin/origin_save: Do not modify unauthorized URLs list when rejecting

When rejecting a Save Code Now request, the submitted origin URL was
automatically added to the unauthorized URLs list. But this was
prolematic as a valid URL prefix could be added in that list and
thus a valid request whose origin URL has that prefix would be
automatically rejected.

So remove the automatic adding of origin URL in unauthorized list
when rejecting a request.

Also return 404 in the reject endpoint when the request does not
exist.