Fixed #21389 -- Accept most valid language codes

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.
Fixed #22056 -- Omitted non-package directories when collecting tests

This allows us to ignore empty dirs or other unrelated dirs.
Thanks Sebastian Pawluś for the report and the initial patch.
Fixed documention typo
Merge pull request #2374 from spaceexperiment/doc_typo

Fixed documention typo
Fixed #22136 -- Updated comment for Textarea widget

Thanks Aymeric Augustin for the suggestion.
Replaced deprecated assertEquals
Fixed #21962 -- Added escape_html flag to ErrorDict.as_json()
Removed Django 1.5 upgrade hints for {% url %} tag.

Refs #19280 and Refs #19392.
Fixed #22129 -- Added steps for creating superusers to tutorial.
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.
Fixed doc typos.
Fix keyword agument names in RunPython to be positional
Fixed #21948 -- Noted TEMPLATE_LOADERS requirement when overriding admin templates.

Thanks django at patjack.co.uk for the suggestion.
Removed some ReST links to undocumented functions.
Added release note and regression test for refs #21643.

This will be backported to stable/1.6.x along with the original fix.
Fixed spelling mistakes in docs.
Added patch_formats utility for i18n formatting tests
Fixed #22137 -- Made Widget.is_hidden a read-only property

Thanks django at patjack.co.uk for the report and the review.
Fixed #22168 -- Fixed migrations failing on sqlite when column names are SQL keywords

Thanks to trac user fallen_flint for the report and initial patch.
Fixed #15318 -- Added settings for language cookie max-age, path, domain

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.