Alerting: modify DB table, accessors and migration to restrict org access (#37414)

* Alerting: modify table and accessors to limit org access appropriately

* Update migration to create multiple Alertmanager configs

* Apply suggestions from code review

Co-authored-by: gotjosh <josue@grafana.com>

* replace mg.ClearMigrationEntry()

mg.ClearMigrationEntry() would create a new session.
This commit introduces a new migration for clearing an entry from migration log for replacing  mg.ClearMigrationEntry() so that all dashboard alert migration operations will run inside the same transaction.
It adds also `SkipMigrationLog()` in Migrator interface for skipping adding an entry in the migration_log.

Co-authored-by: gotjosh <josue@grafana.com>

pkg/services/ngalert/models/alertmanager.go

@@ -10,10 +10,12 @@ type AlertConfiguration struct {
 	ConfigurationVersion      string
 	CreatedAt                 int64 `xorm:"created"`
 	Default                   bool
+	OrgID                     int64 `xorm:"org_id"`
 }
 
 // GetLatestAlertmanagerConfigurationQuery is the query to get the latest alertmanager configuration.
 type GetLatestAlertmanagerConfigurationQuery struct {
+	OrgID  int64
 	Result *AlertConfiguration
 }
 
@@ -22,8 +24,5 @@ type SaveAlertmanagerConfigurationCmd struct {
 	AlertmanagerConfiguration string
 	ConfigurationVersion      string
 	Default                   bool
-}
-
-type DeleteAlertmanagerConfigurationCmd struct {
-	ID int64
+	OrgID                     int64
 }