Disable local accounts when Guisso is enabled

[matiasgarciaisaia]

If we delegate authentication to Guisso, we should stop using Devise's local accounts.

We want to have this in production, since Guisso has CAPTCHA implemented to avoid automated account creation - and we want to avoid implementing CAPTCHA on each of our apps.

There was a failed attempt to implement this in #909, but it broke the app - see bbc073c.