Alerting: Add configurable severity support for PagerDuty notifier (#19425)

* Pagerduty notifier: configurable severity

Instead of hardcoding `critical` make it configurable per notification channel instance.

* fix html

* Add a test to ensure default severity is correct

* Notifications doc

* Add a non-default test

* Add err check on NewJson (all tests)

* Add default severity (critical) to AlertNotificationEditCtrl class

public/app/features/alerting/NotificationsEditCtrl.ts

@@ -19,6 +19,7 @@ export class AlertNotificationEditCtrl {
     settings: {
       httpMethod: 'POST',
       autoResolve: true,
+      severity: 'critical',
       uploadImage: true,
     },
     isDefault: false,