Fixed #11295: If ModelAdmin.queryset returns a filtered QS don't require a 2nd count call

Original patch rewritten, added tests and get_filters_params method for ChangeList class.
Thanks Alex for the report.

Fixes #5568 -- DROP INDEX subcommand 

Add documentation for new command in django-admin.

This reverts commit 2b76f19f.

Change patch from https://code.djangoproject.com/ticket/5568
to work on modern Django.
Add special case for MySQL which has different syntax for DROP INDEX.
Add unit tests for the new functionality.

Fixed #11971 -- Documented the serialization formats

This was missing from f49e9a51.

refs #19889

Thanks to Ciantic for the report, claudep and slurms for initial work

This is ensured through the CSRF protection of the view

fixes #19263 - EmptyResultSet in subquery causes incorrect SQL

Fixed #19609: admin Inlines doesn't display help_text for readonly fields

Made cached_property to behave as property when accessed via class.

Fixed #19526 - make regexes for css case insensitive

CSS specifications governs that syntax is case insensitive.
This modifies CachedFilesMixin to support that.

Thanks facundo.olano at gmail.com for the report and thikonom for
the initial patch.

Thanks wsmith323 for the patch.

Fixes #18162 --  Specifying choices to Field overrides any form_class argument

Thanks pydanny for the report and ptone for the patch.

When iteration over a queryset raised an exception, the result cache
remained initialized with an empty list, so subsequent iterations returned
an empty list instead of raising an exception

Fixes #17866: Vary: Accept-Language header when language prefix used

Thanks almalki for the report.

Thanks monkut for the report and wsmith323 for the patch.