Fixed #11639, #13618 -- Added get_prepopulated_fields method to ModelAdmin and InlineModelAdmin to be able to handle prepopulated fields on a case-by-case basis. Thanks, leanmeandonothingmachine.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16069 bcc190cf-cafb-0310-a4f2-bffc1f526a37