ch17|023 - Replace import to enable easier migration to Django 2

[HelloThisIsFlo]

Instead of importing

from django.core.urlresolvers import reverse

we could import

from django.urls import reverse

Importing from urlresolvers is only kept for backwards compatibility, and it isn't working in Django 2.0 anymore.
See: https://docs.djangoproject.com/en/1.11/ref/urlresolvers/