Add support for importing/exporting avatars

We were trying to import Comentario v1 type of data and found out that avatars are not being imported. It'd be great to be able to import and export avatar data as well.

I understand that the new Comentario v3 data structure actually stores the avatars in the database, so it posses some extra challenges. I did hack my way through the v1 import though, but not sure what's the best approach with export and v3 import would be.