Import all Alertmanager files from grafana/grafana

[gotjosh]

This includes all of the files in the notifier directory of grafana/grafana.

I've run:

 git filter-repo --path pkg/services/ngalert/notifier/ 

and then:

git filter-repo --path-rename pkg/services/ngalert/notifier/:alerting/

There are def more files than what we need here and I'll start deleting right after but wanted to start with a clean slate as opposed to moving each file individually.

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