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/apps.py

@@ -0,0 +1,29 @@
+from __future__ import unicode_literals
+
+from django.apps import AppConfig
+from django.core.cache import caches
+from django.db.utils import ProgrammingError
+from django.core.exceptions import ImproperlyConfigured
+from django.conf import settings
+
+
+class ExtrasConfig(AppConfig):
+    name = "extras"
+
+    def ready(self):
+        import extras.signals
+
+        # check that we can connect to redis
+        if settings.WEBHOOK_BACKEND_ENABLED:
+            try:
+                import redis
+                rs = redis.Redis(settings.REDIS_HOST,
+                                 settings.REDIS_PORT,
+                                 settings.REDIS_DB,
+                                 settings.REDIS_PASSWORD or None)
+                rs.ping()
+            except redis.exceptions.ConnectionError:
+                raise ImproperlyConfigured(
+                    "Unable to connect to the redis database. You must provide "
+                    "connection settings to redis per the documentation."
+                )