Alert- Notifications - Port Additional Notification Types

We are maintaining a few alarm callback plugins that are widely used by users and customers. The important ones should also be ported to the new notification system.

The following plugins might be candidates but we need to check with @kroepke about the priorities:

Slack - https://github.com/graylog-labs/graylog-plugin-slack
PagerDuty - https://github.com/graylog-labs/graylog-plugin-pagerduty
Twilio SMS - https://github.com/graylog-labs/graylog-plugin-twiliosms
Hipchat - https://github.com/graylog-labs/graylog-plugin-hipchat
Jabber - https://github.com/graylog-labs/graylog-plugin-jabber