django migration should respect SOCIAL_AUTH_USER_MODEL setting

[max-arnold]

We use stock auth.User model for administrators (without social auth) and custom UserAccount model for other types of users with enabled social auth. So we have SOCIAL_AUTH_USER_MODEL = 'customauth.UserAccount'.

The patch fixes the following failure:

./manage.py migrate --fake-initial

django.db.utils.IntegrityError: insert or update on table "social_auth_usersocialauth" violates foreign key constraint "social_auth_usersocialauth_user_id_17d28448_fk_auth_user_id"
DETAIL:  Key (user_id)=(225873) is not present in table "auth_user".

[max-arnold]

Do you have any objections against this patch? To me the fix looks obvious (bundled south migrations have something similar: https://github.com/omab/python-social-auth/blob/master/social/apps/django_app/default/south_migrations/__init__.py)

[omab]

Thanks!