Use a Swappable Model for CRONTAB_MODEL

[schinckel]

This is the same pattern used by django.contrib.auth (settings.AUTH_USER_MODEL), and I also implemented it for django-occupation.

https://bitbucket.org/schinckel/django-occupation/src/default/src/occupation/models.py

I also implemented a helper function for getting the model:

https://bitbucket.org/schinckel/django-occupation/src/f1ac9d05e3de436ec786cfe608ba5f15149589a0/src/occupation/utils.py#lines-17

It's been a while, but I think that was the same pattern used by d.c.a