Regression in 58da81a5.

Refs #27985 -- Reallowed using __exact=None as an alias for __isnull=True if a custom lookup class with lookup_name != None is registered as the exact lookup.

Regression in 58da81a5 and prerequisite
for refs #28896.

Refs #28856 -- Added test for caching of a GenericForeignKey pointing to a model that uses more than one level of MTI.

Forwardport of test and release notes of
35222035 from stable/1.11.x

This required allowing WhereNode to be provided as When(condition).

This was made possible by cf12257d.

Fixed #28791 -- Allowed commands that don't require settings to work if the DJANGO_SETTINGS_MODULE doesn't exist.

Marked the whole pattern (e.g. "{value}th") as translatable, instead of
just this suffix, so that languages not using suffixes for ordinals can
use this tag.

Thanks Simon Charette and Tim Graham for the review and Carlos E. C.
Leite for the report.

Refs #28876 -- Fixed incorrect class-based model index name generation for models with quoted db_table.

Thanks Simon Charette and Tim Graham for the review and Carlos E. C.
Leite for the report.

Regression in b52c7300.

Also allowed autocomplete widgets to work on AdminSites with a name other
than 'admin'.