* Add method GetAllLatestAlertmanagerConfiguration to DBStore
* add method ApplyConfig to AlertManager
* update multiorg alert manager to load all alertmanager configs at once
* Alerting: Persist notification log and silences to the database
This removes the dependency of having persistent disk to run grafana alerting. Instead of regularly flushing the notification log and silences to disk we now flush the binary content of those files to the database encoded as a base64 string.
Introduces org-level isolation for the Alertmanager and its components.
Silences, Alerts and Contact points are not separated by org and are not shared between them.
Co-authored with @davidmparrott and @papagian
