* do not suppress NoData state
* extract conversion of state to postable alert + tests
* create a special alert instance if nodata
* use NoData when converting from Keep Last State instead of Alerting
* add silence during migration if NoData is mapped to KeepLastState.
* Alerting: Fix alert flapping in the alertmanager
fixes a bug that caused Alerts that are evaluated at low intervals (sub 1 minute), to flap in the Alertmanager.
Mostly due to a combination of `EndsAt` and resend delay.
The Alertmanager uses `EndsAt` as a heuristic to know whenever it should resolve a firing alert, in the case that it hasn't heard
back from the alert generation system.
Because grafana sent the alert with an `EndsAt` which is equal to the `For` of the alert itself,
and we had a hard-coded 1 minute re-send delay (only applicable to firing alerts) this meant that a firing alert would resolve in the Alertmanager before we re-notify that it still firing.
This commit, increases the `EndsAt` by 3x the the resend delay or alert interval (depending on which one is higher). The resendDelay has been decreased to 30 seconds.
