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/migrations/0012_webhooks.py

@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.10 on 2018-05-23 16:35
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('contenttypes', '0002_remove_content_type_name'),
+        ('extras', '0011_django2'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Webhook',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=150, unique=True)),
+                ('type_create', models.BooleanField(default=False, help_text='A POST will be sent to the URL when the object type(s) is created.')),
+                ('type_update', models.BooleanField(default=False, help_text='A POST will be sent to the URL when the object type(s) is updated.')),
+                ('type_delete', models.BooleanField(default=False, help_text='A POST will be sent to the URL when the object type(s) is deleted.')),
+                ('payload_url', models.CharField(max_length=500, verbose_name='A POST will be sent to this URL based on the webhook criteria.')),
+                ('http_content_type', models.PositiveSmallIntegerField(choices=[(1, 'application/json'), (2, 'application/x-www-form-urlencoded')], default=1)),
+                ('secret', models.CharField(blank=True, help_text="When provided the request will include a 'X-Hook-Signature' header which is a HMAC hex digest of the payload body using the secret as the key. The secret is not transmitted in the request.", max_length=255)),
+                ('enabled', models.BooleanField(default=True)),
+                ('ssl_verification', models.BooleanField(default=True, help_text='By default, use of proper SSL is verified. Disable with caution!')),
+                ('obj_type', models.ManyToManyField(help_text='The object(s) to which this Webhook applies.', related_name='webhooks', to='contenttypes.ContentType', verbose_name='Object(s)')),
+            ],
+        ),
+        migrations.AlterUniqueTogether(
+            name='webhook',
+            unique_together=set([('payload_url', 'type_create', 'type_update', 'type_delete')]),
+        ),
+    ]