Allow logins to existing accounts only with associated accounts

To prevent people can log into existing account with a newly created
social account with a forged email address. We only allow logins with
previously approved service providers.

When a user logs in for the first time, eg. the email does not exists,
then the user is created and the social account is approved
automatically.
send notification mail on new users
added Google (untested)

don't have Internet right now