This adds a notification service to Kibana that can be used to send
asynchronous notifications, such as sending email and Slack messages,
which are intended to be configured via a combination of the
`kibana.yml` and Kibana keystore.
Once configured, the actions are automatically added to the notification
service and can be invoked via the server using the `notificationService`
singleton or HTTP to send it directly. See the included README for more
details.
