Remove usage of django-heroku

[smartspot2]

The django-heroku package is deprecated, as of October 15, 2024. The package doesn't really do too much, other than set a few configuration parameters, which we can definitely do ourselves.

See https://github.com/heroku/python-getting-started/blob/main/gettingstarted/settings.py for an example of a Django settings.py file for use in Heroku. Further, see https://github.com/heroku/django-heroku/blob/master/django_heroku/core.py for the configuration that django_heroku adds, which we should inline in our settings.py.