Fix remaining Django 4.0 warnings

[inducer]

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)