Add checksums module from django.utils.checksums

[Django ticket #23613](https://code.djangoproject.com/ticket/23613) calls out `django.utils.checksums` for being unused and suggest moving it to django-localflavor.

This pull request includes the checksums module from django, the associated testcase and replaces uses of the luhn algorithm in localflavor with calls to the `luhn` function.

I didn't fully comprehend the implementation of the Luhn algorithm `se.utils.id_number_checksum` so I left that one alone.