Add Debian bug reference for CVE-2019-12781/python-django

Please note that I wrongly pasted the CVE for the #931316 resulting in
wrong templated CVE description text. The former CVE-2019-12308 was
already fixed.

Debian bug #931316 is meant to track the CVE-2019-12781 issue as
published in
https://www.djangoproject.com/weblog/2019/jul/01/security-releases/ .