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.

This will be backported to stable/1.6.x along with the original fix.

Thanks django at patjack.co.uk for the suggestion.

Fixed #22028 -- Documented that database signals aren't emitted for apps that laack a models module.

Thanks un33k for the suggestion and donjpacheco for the draft patch.

Refs #19280 and Refs #19392.

Thanks Aymeric Augustin for the suggestion.

Fixed documention typo

This allows us to ignore empty dirs or other unrelated dirs.
Thanks Sebastian Pawluś for the report and the initial patch.

By removing the 'supported' keyword from the detection methods and only relying
on a cached settings.LANGUAGES, the speed of said methods has been improved;
around 4x raw performance. This allows us to stop checking Python's incomplete
list of locales, and rely on a less restrictive regular expression for
accepting certain locales.

HTTP Accept-Language is defined as being case-insensitive, based on this fact
extra performance improvements have been made; it wouldn't make sense to
check for case differences.

Thanks skipjack at peru.com for the suggestion.

Thanks simon29 for the suggestion.

Thanks EvilDMP for the suggestion.

Thanks donjpacheco for the initial patch.

Refs #22015

Fixed #22019 -- added documentation on Model attributes