New port: www/py-dj21-django-auth-ldap

Repo-copy www/py-django-auth-ldap and assign it to Django 2.1 to enable LDAP
authentication with net-mgmt/netbox that has been switched to Django 2.1
recently.

Also limit the port to Python 3.5+ as Django 2.1 requires it as minimum
dependency and to save some overhead in the building infrastructure.

Reported by: Thomas Kurschel (via private email)
Approved by: miwi (mentor)
Differential Revision: https://reviews.freebsd.org/D19576