Can't migrate database on fresh install with cookiecutter-django.

[flaviobarros]

Actual behavior

I have just created a basic project with cookiecutter-django. After having a working site I tried to install zinnia following documentation but It doens't migrate the database. I receive the following error:

LookupError: Model 'users.User' not registered.

Expected behavior

The migration could be done without any error.

Steps to reproduce the issue

1. Install cookiecutter-django basic project (Mailgun, Heroku and etc). After having a working site
2. Install zinnia following documentation
3. try migrate

python manage.py migrate

Specifications

• Zinnia version: 0.20
• Django version: 2.0.4
• Python version: 3.5.2
• Operating system: Linux Mint 18.2

Disclaimer

Before submitting an issue make sure you have:

• [x] Read the guidelines for contributing.
• [x] Checked for duplicate issues.
• [x] Not a support request.

[flaviobarros]

I have tested with cookiecutter-django multiple versions, using django 1.11, 1.10 and 1.9. Didn't work with any of them.

[Admoroux]

In zinnia/models/author.py:

def safe_get_user_model():
    """
    Safe loading of the User model, customized or not.
    """
    user_app, user_model = settings.AUTH_USER_MODEL.split('.')
    return apps.get_registered_model(user_app, user_model)

I changed it as:

[...]
from django.contrib.auth import get_user_model
[...]

def safe_get_user_model():
    """
    Safe loading of the User model, customized or not.
    """
    # user_app, user_model = settings.AUTH_USER_MODEL.split('.')
    # return apps.get_registered_model(user_app, user_model)
    return get_user_model()

and runs perfectly.

I'm trying to test it in a clean installation. If it works, i'll make a pull request with it.