Create a new Mk/Uses/django.mk (largely copied from lua.mk) which adds
a RUN_DEPENDS on the users preferred choice of Django version. Choose
from 1.11, 1.10 or 1.8 currently. Also generate a
DJANGO_PKGNAMEPREFIX which appends the django version to
PYTHON_PKGNAMEPREFIX (so the final result is eg. py27-django118- )
Add a DJANGO_DEFAULT setting to bsd.default-versions.mk
Set the ports default to django-1.11, which is the upstream
recommended default, rather than the current django-1.8
Convert all the ports that have a run-time dependency on Django to
USES+=django
While here convert one port to options helpers and (not really
relevant for this) update another as well as converting to the new
USES+=django