Fixed FB login.

The main problem was the configuration on FB.

I also changed the callback handler to return any user with the given
email, no matter how they may have logged in in the past, and I did a
little refactoring.

I also re-enabled the FB login icon.
Removed the FB authentication from the configuration.
Update the provider everytime a user logs in and use that avatar.

This is so we get the FB avatar whenever the user logs in from FB.