Updated main_app/models.py main_app/views.py to work with Django v2

[Pax-nl]

Updated models.py and views.py to work with Django v2. Updated settings.py to handle the new way of using MIDDLEWARE to fix

'WSGIRequest' object has no attribute 'session'

https://docs.djangoproject.com/en/2.0/topics/http/middleware/#activating-middleware