publish release event notifications

[bhearsum]

The services operations team would like to track Fx release events on our various dashboards to see how client releases influence server traffic.

The specific use case is that ServiceOps wants to be able to overlay release shipping and throttling events on traffic graphs for other services in their graphs. The relevant information is the Background Rate, Firefox version being shipped, and the timestamp of the change. We already store all of this in Balrog's history tables, so the main thing here is getting it pushed to a useful place.

At this time, the Grafana Annotations API (https://grafana.com/docs/grafana/latest/reference/annotations/) is the best way to do this, although the idea of RSS or something pullable has been talked about too.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1376537)

[bhearsum]

The right place to do this has probably changed since this was filed. We should talk to stakeholders again before proceeding here to make sure we publish the right information in the right ways. (@Micheletto is one, there are probably others)