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.

Typo fixes, spell check, some more specifics where possible.

Thanks to James for the first draft; I made a few changes (svn->git) and
some supporting links, but mostly I added FIXME's.

Thanks ddavies at nomensa.com for the report and the patch and
serdaroncode for reviewing the patch.

Changed %r to %s in get_language_info error message

Thanks theaspect at gmail.com for the report and volrath for the
patch.

Refactoring: field was renamed to field_name, since flatten_fieldsets returns field name, not field.

Original patch from Marc Aymerich Gubern

Fixed #19854 -- Added test runner option to skip Selenium tests

Thanks Kronuz for the report and the initial patch.

Fixed #19811 - Added language code fallback in get_language_info

Updated example to use staticfiles static template tag, thanks reinout for the suggestion, refs #19675.

Fixed #18899 -- FileSystemStorage.save should support any file-like objects