This code added in 45e04993
is no longer necessary to pass the model_formsets_regress tests
as of 5e2c4a4b.

With the introduction of multiple template engines these exceptions are no
longer DTL-specific. It makes more sense for them to be moved out of
DTL-related modules.

Thanks Michael Diener for the report and sample data, and Tim Graham
for the review.

This was fixed earlier but only when the M2M relation was at the first
level on the object. This commit fixes the issue even when the M2M is
at deeper levels, such as behind a foreign key.

Fixed #24564 -- Moved AbstractBaseUser and BaseUserManager so they can be used without auth in INSTALLED_APPS

Case expressions weren't copied deep enough (self.cases list was
reused resulting in an error).

Refs #24733.

The inner import was dropped during the reorganizing of the database
backends in commit 28308078.

Thanks Preston Timmons for the report.

Only compute the CSRF_COOKIE when it is actually used. This is a
significant speedup for clients not using cookies.

Changed result of the “test_token_node_no_csrf_cookie” test:  It gets
a valid CSRF token now which seems like the correct behavior.

Changed auth_tests.test_views.LoginTest.test_login_csrf_rotate to
use get_token() to trigger CSRF cookie inclusion instead of changing
request.META["CSRF_COOKIE_USED"] directly.

When "Save as new" is chosen and errors occur, only show the "Save as new"
button and not the other save buttons.

Thanks to Tino de Bruijn for doing the real work on this fix.

This avoids calling date.tzinfo.utcoffset(date) twice. It's also
robust to cases where that function returns None -- which never
happens in practice :-)

Thanks Aymeric Augustin for the review.