Alerting: Persist rule position in the group (#50051)

Migrations:
* add a new column alert_group_idx to alert_rule table
* add a new column alert_group_idx to alert_rule_version table
* re-index existing rules during migration

API:
* set group index on update. Use the natural order of items in  the array as group index
* sort rules in the group on GET
* update the version of all rules of all affected groups. This will make optimistic lock work in the case of multiple concurrent request touching the same groups.

UI:
* update UI to keep the order of alerts in a group

pkg/services/sqlstore/migrations/migrations.go

@@ -92,6 +92,8 @@ func (*OSSMigrations) AddMigration(mg *Migrator) {
 	accesscontrol.AddManagedFolderAlertActionsMigration(mg)
 	accesscontrol.AddActionNameMigrator(mg)
 	addPlaylistUIDMigration(mg)
+
+	ualert.UpdateRuleGroupIndexMigration(mg)
 }
 
 func addMigrationLogMigrations(mg *Migrator) {