text: Title: allauth_settings USER_MODEL_EMAIL_FIELD not taken into consideration
  Body: I am implementing dj-rest-auth to only add social log-in to my rest framework and my User model has an email field that has different name than "email". Therefore I set the USER_MODEL_EMAIL_FIELD but this is not taken into consideration in the serializers. 
  
  When trying to do social login of an account that already exists signed by email the code breaks. I made a quick fix changing [this part](https://github.com/Brechard/dj-rest-auth/blob/8a460ecf9a72aec269b75160e5c97f7ed608e247/dj_rest_auth/registration/serializers.py#L161) by this:
  
      email = getattr(login.user, allauth_settings.USER_MODEL_EMAIL_FIELD)
      email_search = {allauth_settings.USER_MODEL_EMAIL_FIELD: email}
      account_exists = get_user_model().objects.filter(**email_search).exists()
  
  This probably should have a bigger effect in the repo and that is why I haven't done a pull request for it
  