feat(oauth): Support for URN redirects


Fixes:  https://github.com/mozilla/fxa-content-server/issues/1585

@ckarlof @zaach r?
This is correct, but not sure if it's the check we want. I think a better check is: isHTTPURI() which will only make it a link if it's an URI that the user can follow. If for some reason we have URIs that are some other non-HTTP URIs then we don't want to create links for them either.