Fixed #12095 - login and other contrib views failing if template rendered using inclusion tag.

The {% csrf_token %} tag is unable to get its value if a template is
rendered using an inclusion_tag, since that creates a brand new Context,
rather than using the existing one.  Since this is a common pattern, and we
need CSRF protection to be as simple and easy as possible, we special case
the csrf_token and copy it from the parent context to the new context.

A more elegant and general solution may appear in future, but this is good
enough for now.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11672 bcc190cf-cafb-0310-a4f2-bffc1f526a37