Stopped the admin JavaScript code from completely removing the `jQuery` object from the global namespace to ease the use of jQuery plugins. The `django.jQuery` object is still supposed to be used by Django's own JavaScript files. Refs #12882.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16415 bcc190cf-cafb-0310-a4f2-bffc1f526a37