Alerting: Keep track of individual org migration status (#78369)

* Alerting: Keep track of individual org migration status

Save migration status per migrated org.

Change the meaning (and key/value) of the org_id=0 entry 
to store the current (previous) config value used by alerting. 
This is so we can know when to upgrade/downgrade by 
comparing with the new config value in 
UnifiedAlerting.IsEnabled.

pkg/services/sqlstore/migrations/migrations.go

@@ -111,6 +111,8 @@ func (*OSSMigrations) AddMigration(mg *Migrator) {
 	dashboardFolderMigrations.AddDashboardFolderMigrations(mg)
 
 	ssosettings.AddMigration(mg)
+
+	ualert.CreateOrgMigratedKVStoreEntries(mg)
 }
 
 func addStarMigrations(mg *Migrator) {