[1.2.X] Added explanatory note on CSRF failure page for the case of a missing Referer header.

  
This is intended to help power users who have disabled Referer headers, or
installed add-ons which have done so, and to help web site administrators
with debugging, since this problem will be browser specific and not a
programming error.

Backport of [13680] from trunk. Technically this is a (tiny) new feature,
but it has been backported because it might give significant help with
debugging rare problems with Django 1.2's new CSRF protection.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13682 bcc190cf-cafb-0310-a4f2-bffc1f526a37