[1.2.X] Prevented non-admin users from accessing the admin redirect shortcut.

If the admin shortcut view (e.g. /admin/r/<content-type>/<pk>/) is
publically-accessible, and if a public users can guess a content-type ID
(which isn't hard given that they're sequential), then the redirect view could
possibly leak data by redirecting to pages a user shouldn't "know about." So
the redirect view needs the same protection as the rest of the admin site.

Thanks to Jason Royes for pointing this out.

Backport of [15639] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15640 bcc190cf-cafb-0310-a4f2-bffc1f526a37