[FEATURE] Support Gravatar

Currently Comentario has the ability to manually update avatars, however I see that most people rarely actively register an account and then upload an avatar themselves.

If possible, I would like Comentario to support Gravatar

Can be processed a bit more, images from Gravatar will be put into /api/users/ for example, to reduce requests from Gravatar

In my opinion, when v3 is stable, this feature can be added