Fixed #11457: tightened the security check for "next" redirects after logins.

      The new behavior still disallows redirects to off-site URLs, but now allows
redirects of the form `/some/other/view?foo=http://...`.

Thanks to brutasse.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12635 bcc190cf-cafb-0310-a4f2-bffc1f526a37