Fixed #16192 -- Made unique error messages in ModelForm customizable.

Overriding the error messages now works for both unique fields, unique_together
and unique_for_date.

This patch changed the overriding logic to allow customizing NON_FIELD_ERRORS
since previously only fields' errors were customizable.

Refs #20199.

Thanks leahculver for the suggestion.
Fixed #21417 -- Expanded TEMPLATE_STRING_IF_INVALID in blocktrans

Thanks keturn for the reporti, Chris Medrela for details and
Tim Graham for the review.
Refs #19915.
Added tests for LocalMemCache deadlocks. refs #20613 and refs #18541.

Thanks Zach Smith for the patch.
Fixed #21237 -- Added Australian English (en_AU) to locale conf

Adds Australian English (en_AU) to locale conf, which differs from
American English in date formatting and spelling, and continues
to diverge from British English in spelling (and first-day-of-week
according to the current en_GB locale format.py).

The transifex language project to match this patch has been requested.
Allowed more easily subclassing of BlockNode tags.
Removed extra backquote in field docs
Fixed #21674 -- Deprecated the import_by_path() function in favor of import_string().

Thanks Aymeric Augustin for the suggestion and review.
Fixed #21959 -- Handled Inf/NaN in widthratio tag.

Thanks rmoe for the report and the patch.
Fixed #19373 -- Ported Windows file locking from PyWin32 to ctypes

There wasn't any file locking under Windows unless PyWin32 was
installed. This removes that (undocumented) dependency by using ctypes
instead.

Thanks to Anatoly Techtonik for writing the ctypes port upon which this
is based.