Use split for django_versions so it's easier to update tests and other version related things

[jonathan-s]

Description

Updating versions is a pain, and it's something that needs to be done on a semi-regular basis. The ideal would be to only need to update cookiecutter.json with all supported versions for the cookiecutter template. The tests should capture if any other changes are needed.

To be able to do this reliably, we need to use a filter split in the jinja templates. This relies on the following PR: cookiecutter/cookiecutter#1240, as split is not part of built-in filters in Jinja as described here: pallets/jinja#898