avatars should not rely on @cpan.org addresses

[haarg]

Delivery of @cpan.org addresses is getting worse and worse, in part because providing redirect emails in that form is very problematic in terms of spam protection.

Currently, we only use @cpan.org address to provide avatars via Gravatar.

This should be changed in some fashion to allow gravatars using the author's public email, or possibly using GitHub avatars. We previously allowed arbitrary URLs, but I think we should stick to some specific external services.