Fix get User model for cookiecutter-django

[Admoroux]

This change fixes issue Can't migrate database on fresh install with cookiecutter-django. #537, opened by flaviobarros. This way, User model is allways notified right using Django Auth system.

What is the purpose of your pull request?

• [X] Bug fix
• [ ] New feature

Proposed changes

Warning

Before submitting a pull request make sure you have:

• [X] Read the guidelines for contributing.
• [ ] Wrote some tests.
• [X] Respected the PEP 8.

I didn't write any test but main zinnia tests pass al OK.

[Oluwaseun241]

Can I contribute?