FEATURE: Add a webhook for user notifications

If enabled, this will fire a webhook whenever a user's notification has
been created. This could potentially be a lot of data depending on your
forum, and should be used carefully since it includes everything all users
will see in their feeds.

db/fixtures/007_web_hook_event_types.rb

@@ -44,3 +44,8 @@ WebHookEventType.seed do |b|
   b.id = WebHookEventType::REVIEWABLE
   b.name = "reviewable"
 end
+
+WebHookEventType.seed do |b|
+  b.id = WebHookEventType::NOTIFICATION
+  b.name = "notification"
+end