Upgrade to Django 2.0.1 & small improvments, fixes & update dependencies

requirements/base.in

requirements/development.in

requirements/development.txt

requirements/production.in

requirements/production.txt

src/homework/migrations/0005_auto_20180126_0135.py

src/homework/models.py

src/homework/permissions.py

src/news/migrations/0003_auto_20180126_0135.py

src/news/models.py

src/news/permissions.py

src/news/serializers.py

src/news/urls.py

src/stats/urls.py