Fixed #16050 -- BACKWARDS-INCOMPATIBLE CHANGE: Moved static files of the admin to conventional file system location.

This also removes the need for ADMIN_MEDIA_PREFIX and replaces it with the convention to find admin's static files at STATIC_URL + 'admin/'.

Thanks to Jacob for the review and general help.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16487 bcc190cf-cafb-0310-a4f2-bffc1f526a37