Fixed #12339 -- Made content type deletion an interactive process to prevent...

Fixed #12339 -- Made content type deletion an interactive process to prevent accidentally cascade deleting content from a production database. Thanks to kcarnold for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12782 bcc190cf-cafb-0310-a4f2-bffc1f526a37