consider option to opt-out of emails

We currently send an email to all uploaders whenever a package is published.

This is largely a security feature to help detect compromises. We can't possibly stop all attacks, but if users can detect attacks and report them that is fantastic, as it enables us to find out how people were compromised and, thus, consider mitigating the attack vector.

But we could consider letting users opt-out of this... On other hand, forcing users to receive emails is a security feature.