[NEW PORT] www/py-django-happenings-update

• Repo-copy from www/py-django-happenings because it's an official fork and also available at PyPI. The forked version has support for newer Django releases.

• Update CONFLICTS_INSTALL entries accordingly but omit www/py-django-mptt as it can co-exist (once assigned to Django 2.2) with www/py-django-happenings-update.

PR: 245644
Approved by: Kevin Golding <ports@caomhin.org> (maintainer)