Fixed #22426 -- Added support old-style d.c.messages format.

Forward ported code from 1.5 that adds backwards compatibility with legacy message length.
See commit 9e7183073f64e541587e8dcfd8bb3ddeb47f8162 for details.

Thanks to Ofir Ovadia for the initial patch.
Added 1.6.4 release note stub.
Use the stdlib's compare_digest for constant time comparisons when available
Merge pull request #2600 from alex/builtin-constant-time-compare

Use the stdlib's compare_digest for constant time comparisons when available
Updated grammar in description of django.contrib.auth.
Various documentation typo/spelling fixes

Errors detected by Topy (https://github.com/intgr/topy), all changes
verified by hand.
Merge pull request #2602 from intgr/typofix

Various documentation typo/spelling fixes