Added back save_model that was removed from [8273] to allow for both pre- and...

Added back save_model that was removed from [8273] to allow for both pre- and post- operations around save in ModelAdmin.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8307 bcc190cf-cafb-0310-a4f2-bffc1f526a37