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

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

Original patch from Marc Aymerich Gubern
Added an example of "default" database dictionary left blank; refs #19775.

Thanks wsmith323 for the patch.
Some style fixes for stuff that was clearly brough tover from doctests.
Fixed #18829 -- Fixed ModelChoiceIterator length

Thanks facundo.olano at gmail.com for the report and thikonom for
the initial patch.
Fixed #19526

CSS specifications governs that syntax is case insensitive.
This modifies CachedFilesMixin to support that.
Merge pull request #809 from asendecka/ticket19526

Fixed #19526 - make regexes for css case insensitive
Fixed #19872

Made cached_property to behave as property when accessed via class.
Fixed #18491 -- deleting a proxy doesn't show warning about cascade deletes
Merge branch 'ticket19872' of https://github.com/oinopion/django
Merge pull request #778 from viciu/19609

Fixed #19609: admin Inlines doesn't display help_text for readonly fields
fixes #19263
Merge pull request #813 from HiddenData/ticket-19263

fixes #19263 - EmptyResultSet in subquery causes incorrect SQL
Sorry, accidental commit, Thanks claudep for catching it!
Removed superfluous cookie check from auth login.

This is ensured through the CSRF protection of the view
Fixed #15198 -- pass request to AuthenticationForm

Thanks to Ciantic for the report, claudep and slurms for initial work
Various tweaks and additions to 'how to release Django' document.
Made a small clarification in tutorial.

refs #19889