text: Title: settings.ACCOUNT_AUTHENTICATION_METHOD MANDATORY Phone Number with Verification
  Body: Currently when I try to set a CustomUser without username field and use USERNAME_FIELD = 'phone_number'
  I cannot login to django admin because allauth's authentication backend tries to search the login user with 'username field'
  
  setting ACCOUNT_USER_MODEL_USERNAME_FIELD = 'phone_number'
  enables admin login.
  However it is still hard to setup the social login functionality with mandatory phone number verification with allauth.
  I think there should be mountable PhoneNumberAuthBackend, etc.