[Feature Request] Notification Channel with multiple destinations

[ShiftedMr]

Please include this information:

What Grafana version are you using? Master
What datasource are you using? Graphite
What OS are you running grafana on? Ubuntu 16.04
In alerting list allow selecting a notification channel as a filter so you can add an extra notification channel to each.
Example: The team has a critical alert chat room BUT we also need to alert the 24/7 monitoring team. This requires 2 notification channels on the same set of alerts. It would be easier to pair the destinations under the same channel

[melor]

Duplicate of #7832