Fixed #4604 - Configurable message passing system, supporting anonymous users

This deprecates User.message_set in favour of a configurable messaging
system, with backends provided for cookie storage, session storage and
backward compatibility.

Many thanks to Tobias McNulty for the bulk of the work here, with
contributions from Chris Beaven (SmileyChris) and lots of code review from
Russell Keith-Magee, and input from many others.  Also credit to the authors
of various messaging systems for Django whose ideas may have been pinched
:-)



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11804 bcc190cf-cafb-0310-a4f2-bffc1f526a37