text: Title: Option to allow one-time password login for old users when using SOCIALACCOUNT_ONLY
  Body: One of my sites would like to migrate to allowing login only using social accounts, but has existing users with passwords.
  
  I think it would be good to have a configuration with the concept of a "legacy password login" option which allows users to authenticate with their password, then migrate to a social account.
  
  I'd be happy to look at doing the work for this, but wanted to raise an issue to see if there was appetite to accept this before I do.