Only validate New URL in whitelist for redirects

Other validations of New URL are only done if the type is a redirect. This means that if you typed in something invalid for New URL and then changed the type to Archive, it won't reject the change you actually wanted and complain about the value of a field which we have hidden from view with JavaScript.

Mad propz to @jennyd for spotting this.
For extra clarity and reminderizing, I'd find it helpful if this test also asserted that the new URL is still saved even though it wouldn't be valid.