Cleanup django_sessions periodically

[oyvindkolbu]

django_sessions should be cleaned up regulary according to django.

Either the django-manage.py clearsessions can be added to a cronjob or have the functionality replicated in navclean.

[lunkwill42]

Since navclean has been refactored quite a bit in master, I don't want to go through the hassle of adding this functionality to 4.7.x only to have to write it all over again in the next release. Adding a cron job for django-manage.py clearsessions will probably suffice.

[lunkwill42]

Well, getting django-admin to work under NAV is a pain, since NAV has a strained relationship with Django's way of doing things. We may need to reimplement after all. The django-admin implementation is here: https://github.com/django/django/blob/1.7.11/django/contrib/sessions/management/commands/clearsessions.py