Gravatar support for imported accounts

Extension of #33 (closed).

During Commento/Comentario import, when Gravatar is enabled in the dynamic config and the email is known, its avatar should be fetched, too.

This has to happen in a sequential fashion, in the background, to prevent hitting Gravatar throttling limits, and the local memory from being exhausted.

For Disqus it doesn't appear to be possible because its import dump doesn't disclose emails.