[1.6.x] 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.

Backport of f286721f7fdc2202f77a5f4d650d9d0779b86811 from master
[1.6.x] Post release version bump.
[1.6.x] Added 1.6.4 release note stub.

Backport of 9fb95dfc9f from master
[1.6.x] Updated grammar in description of django.contrib.auth.

Backport of 9853779805 from master
[1.6.x] Various documentation typo/spelling fixes

Errors detected by Topy (https://github.com/intgr/topy), all changes
verified by hand.

Backport of 11d453bcadad3c9a5ae35882792d0f1e8e8e7818 from master