Fixed #22145 -- Clarified docs on Field.blank and Field.null options.

Thanks skipjack at peru.com for the suggestion.
Fixed bad grammar in a comment.
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