Fix remaining Django 4.0 warnings

As of f6710f73, the following remain:
```
/home/andreas/work/django/relate/.venv/lib/python3.9/site-packages/django/apps/registry.py:91: RemovedInDjango41Warning: 'course' defines default_app_config = 'course.apps.CourseConfig'. Django now detects this configuratio
n automatically. You can remove default_app_config.
/home/andreas/work/django/relate/.venv/lib/python3.9/site-packages/django/apps/registry.py:91: RemovedInDjango41Warning: 'accounts' defines default_app_config = 'accounts.apps.AccountsConfig'. Django now detects this config
uration automatically. You can remove default_app_config.
/home/andreas/work/django/relate/tests/test_auth.py:2061: RemovedInDjango40Warning: django.conf.urls.url() is deprecated in favor of django.urls.re_path().
url(r"^course"
/home/andreas/work/django/relate/tests/test_auth.py:2066: RemovedInDjango40Warning: django.conf.urls.url() is deprecated in favor of django.urls.re_path().
url(r"^course"
/home/andreas/work/django/relate/tests/test_auth.py:2076: RemovedInDjango40Warning: django.conf.urls.url() is deprecated in favor of django.urls.re_path().
url(r"^course"
/home/andreas/work/django/relate/course/auth.py:258: RemovedInDjango40Warning: request.is_ajax() is deprecated. See Django 3.1 release notes for more details about this deprecation.
if not request.is_ajax() or request.method != "POST":
/home/andreas/work/django/relate/.venv/lib/python3.9/site-packages/django/utils/functional.py:228: RemovedInDjango40Warning: django.utils.http.urlquote() is deprecated in favor of urllib.parse.quote().
return func(*args, **kwargs)
```
(plus a few in third-party dependencies)

Start with the warning locations in course/auth.py and tests/test_auth.py, then locate the app configuration for course and accounts that emits the default_app_config warnings. Run the relevant test suite or warning-producing command first; done means the listed project warnings no longer appear, while third-party dependency warnings may remain.

Written by the indexing model from the issue text.