Fixed #21893 -- ModelState didn't account for MTI parents inherited from abstract models.

This feature allows the default `TIMEOUT` Cache argument to be set to `None`,
so that cache instances can set a non-expiring key as the default,
instead of using the default value of 5 minutes.

Previously, this was possible only by passing `None` as an argument to
the set() method of objects of type `BaseCache` (and subtypes).

The original patch for custom prefetches didn't allow usage of custom
queryset for single valued relations (along ForeignKey or OneToOneKey).
Allowing these enables calling performance oriented queryset methods like
select_related or defer/only.

Thanks @akaariai and @timgraham for the reviews. Refs #17001.

Thanks django-issues at colons.co for the suggestion.

Thanks matt at schinckel.net for the suggestion.

Thanks Piotr Kasprzyk for help with the patch.

Allows custom behavior for setting initial form data in ModelAdmin.
By default, initial data is set via GET params. The new method allows
this behavior to be overridden.

Thanks egasimus for the suggestion.

Thanks to Thomas Güttler for the suggestion.

Thanks to Rodolfo Carvalho and Piotr Kasprzyk
for the patch.

Fixed #22130 -- fixed template_tests/defaultfilters order dependent test failure

Thanks EmilStenstrom for the suggestion.

Fixed #22130 -- Deprecated fix_ampersands and clean_html.

Introduced a number of settings to configure max-age, path, and domain
for the language cookie: LANGUAGE_COOKIE_AGE, LANGUAGE_COOKIE_PATH and
LANGUAGE_COOKIE_DOMAIN.

Thanks sahid for the suggestion.

Thanks to trac user fallen_flint for the report and initial patch.

Thanks django at patjack.co.uk for the report and the review.