Implements #81 - webhook event backend (#1640)

* merge branch develop

* bugfix, signals for virtualization's class wasn't correctly defined

* updated webhooks for 2.4 and cleanup

* updated docs to cover changes to supervisor config

* review changes and further cleanup

* updated redis connection settings

* cleanup settings

netbox/extras/__init__.py

@@ -0,0 +1,15 @@
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+
+
+default_app_config = 'extras.apps.ExtrasConfig'
+
+# check that django-rq is installed and we can connect to redis
+if settings.WEBHOOK_BACKEND_ENABLED:
+    try:
+        import django_rq
+    except ImportError:
+        raise ImproperlyConfigured(
+            "django-rq is not installed! You must install this package per "
+            "the documentation to use the webhook backend."
+        )