Disable local accounts when Guisso is enabled #926
Description
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.