Fixed #17521 -- Made the minified JS files be used in the admin only when...

Fixed #17521 -- Made the minified JS files be used in the admin only when `DEBUG` is `False`. Thanks to Travis Swicegood for the suggestion and patch, and to viciu for the tests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17454 bcc190cf-cafb-0310-a4f2-bffc1f526a37