Default placeholder image for users

[Enrico Piccini]

The default image of users must be "/static/images/user_avatar.png" and not the gravatar. We need to check in code for this... (I opened an issue also in the front-end repo)

[André Palma]

This is now done. However, users will have to invalidate their images on gravatar. The real solution here should be try to use facebook images for user profiles.