gnu: Django packages no longer propagate Django.

gnu: Django packages no longer propagate Django.

This makes them easier to use with different versions of Django.

(python-django-classy-tags, python-django-debug-toolbar-alchemy,
python-django-jinja, python-django-bulk-update,
python-djangorestframework)[propagated-inputs]: Remove PYTHON-DJANGO.
(python-django-contrib-comments)[propagated-inputs]: Likewise.  Add
PYTHON-SIX.
[native-inputs]: Add PYTHON-DJANGO.
* gnu/packages/django.scm (python-django-extensions,
python-django-simple-math-captcha, python-django-taggit,
python-easy-thumbnails, python-django-haystack, python-django-allauth,
python-django-debug-toolbar, python-django-assets, python-django-picklefield,
python-django-contact-form), python-django-pipeline, python-django-redis,
python-django-rq, python-django-q, python-django-sortedm2m,
python-django-appconf, python-django-statici18n, python-django-sekizai,
python-django-crispy-forms, python-django-override-storage,
python-django-auth-ldap, python-django-logging-json, python-django-netfields,
python-django-url-filter)[propagated-inputs]: Move PYTHON-DJANGO ...
[native-inputs]: ... here.