Migrate to django 2.2.4

The django.core.urlresolvers module is removed in favor of its new location, django.urls.
The on_delete argument for ForeignKey and OneToOneField is required in models and migrations.

instances/views.py

@@ -8,7 +8,7 @@ import string
 import random
 from bisect import insort
 from django.http import HttpResponse, HttpResponseRedirect
-from django.core.urlresolvers import reverse
+from django.urls import reverse
 from django.shortcuts import render, get_object_or_404
 from django.utils.translation import ugettext_lazy as _
 from django.contrib.auth.decorators import login_required