Refactor post_save signal for users

Ticket #1127 correctly mentioned that we can't use Django's
get_user_model to register signals:
https://docs.djangoproject.com/en/1.6/topics/auth/customizing/#custom-users-and-signals

This commit rewrites the affected migrate_alerts_to_user receiver to
just happen in Oscar's AbstractUser save() method. This works upwards of
Django 1.4. For Django 1.4, a receiver must be manually registered to
the correct User model.