Clean South app migration reset

To selectively reset your migrations for one app.
Verify that all your environnements are up to date:

rm <app-dir>/migrations/*
python manage.py schemamigration <app-name> --initial
python manage.py migrate <app-name> 0001 --fake --delete-ghost-migrations

then run python manage.py migrate <app-name> 0001 --fake --delete-ghost-migrations on other environnements