This avoids duplicating code.

To resolve the conflict between application names.

Now that the refactorings are complete, it isn't particularly useful any
more, nor very well named. Let's keep the API as simple as possible.

Fixed #21689.

Many thanks to @bitdancer and @aaugustin for answering my stupid questions about (mail)encodings and pointing me in the right direction.

Thanks ashley at ashleymills.com for the suggestion.

Changed console and filebackend to use msg.as_bytes to output the data as it would get send via smtp.

Thanks Keryn Knight for the report.

After the recent series of refactorings, there's no reason to keep
two distinct methods.

Refs #21681.

To the best of my understanding, since populate_models() is now called
as soon as Django starts, it cannot be called while a models module is
being imported, and that removes the need for postponing.

(If hell breaks loose we'll revert this commit.)

Refs #21681.

Since it triggers imports, it shouldn't be done lightly.

This commit adds a public API for doing it explicitly, django.setup(),
and does it automatically when using manage.py and wsgi.py.

Refs #1796, #21676.

Currently it's 3.3.

Various small fixes while I was proof-reading.

Use app_config.get_models() instead.

This avoid leaking implementation details to tests that swap models.

Thanks Simon.

TOOOOO MUCH GLOBAL STATE (you didn't see that).

Thanks Simon for the suggestion.

Also removed inappropriate lowercasing of app labels in migrations.
Unlike model names, they are case sensitive.

Thanks Florian for isolating the shortest way to reproduce this issue:

./runtests.py \
    django.contrib.auth.tests.test_context_processors.AuthContextProcessorTests.test_perms_attrs \
    django.contrib.auth.tests.test_auth_backends.ChangedBackendSettingsTest.test_changed_backend_settings \
    django.contrib.auth.tests.test_auth_backends.CustomUserModelBackendAuthenticateTest.test_authenticate \
    django.contrib.auth.tests.test_basic.BasicTestCase.test_createsuperuser_management_command