Remove gravatar dependency on @cpan.org

[haarg]

The @cpan.org mail redirector is very unreliable, and unlikely to be improved. We should move away from using it.

Users are able to provide email addresses. We have some infrastructure for calculating a gravatar based on that. But it is possible to overwrite it, and it doesn't get updated if the email is changed. It should instead always be calculated based on the email and not allow custom URLs.

We could also consider leveraging GitHub avatars rather than using gravatars.

Alternatively, we could allow users to upload their own avatars. This would require more infrastructure though.