Implement a new notification type named PagerDuty.

This notification defines a PagerDuty action that can send events
to PagerDuty through its Events API.

The UI for this notification provides a combo box where clients
can easily select which PagerDuty service they want events sent
to.

This combo box is automatically populated by providing an API access
key and PagerDuty subdomain in Advanced... PagerDuty Settings.