Olefloch/ops 935/display maintenance page

To test, if `django-short-urls` still works locally for you (it doesn't for me, opened https://work4labs.atlassian.net/browse/OPS-1490 ), you can run `./manage.py runserver` and then

  - load http://127.0.0.1:8000/foobarbla to trigger the new 404 page
  - break the code and reload to trigger the new error 500 page

Make sure `DEBUG = False`!