Added IE conditional comments to contrib.admin base template to block CSS from...

Added IE conditional comments to contrib.admin base template to block CSS from IE5 and lower and serve a patch to IE6 with IE conditional comments. Removed dependency on CSS hacks and empty null.css file.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9300 bcc190cf-cafb-0310-a4f2-bffc1f526a37