Consider designing a redirector service for pub.dev

To mask `referrer` header, we should consider making external links go through a redirect. This also hardens a few other XSS vectors.

See: https://en.wikipedia.org/w/index.php?title=URL_redirection&oldid=917753021#Referrer_masking

Note: It is critical that such a service only allows redirecting URLs embedded on pub.dev, and cannot be used for arbitrary URL redirection. Hence, URLs would need signing or something else.