Fix broken autoload of django-admin-index fixture

[swrichards]

The django-admin-index fixture requires all contenttypes to be
present before being loaded. Previously, we would only ensure
the contenttypes for our own apps were loaded, but this is not
sufficient, as the fixture frequently makes references to third
party apps. This commit clarifies this behavior and ensures that
all contenttypes are loaded.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.83333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 95.11%. Comparing base (c891aba) to head (363f0db).

Files	Patch %	Lines
src/open_inwoner/accounts/apps.py	37.50%	5 Missing ⚠️
...inwoner/accounts/tests/test_admin_index_fixture.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1307      +/-   ##
===========================================
- Coverage    95.13%   95.11%   -0.02%     
===========================================
  Files          983      984       +1     
  Lines        35847    35868      +21     
===========================================
+ Hits         34104    34117      +13     
- Misses        1743     1751       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.