* Alerting: Notification history query API.
First cut at defining a namespace scoped route on the historian.alerting app
to query notification history.
* Address review comments
We have two historians in alerting - alert state and notification. The intention
of this app is to provide query capabilities for both.
In this initial commit, the existing /history API is simply cloned to the new
app. It is identical except that it will send Kubernetes-style error responses
instead of Grafana-style.
This approach was taken to implement the new app more iteratively - ideally we
would define a new API, but this requires quite a significant overhaul of the
backend code.
Steve Simpson