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/ngalert/api/api_prometheus.go

@@ -191,7 +191,7 @@ func (srv PrometheusSrv) toRuleGroup(groupName string, folder *models.Folder, ru
 		Name: groupName,
 		File: folder.Title, // file is what Prometheus uses for provisioning, we replace it with namespace.
 	}
-
+	ngmodels.RulesGroup(rules).SortByGroupIndex()
 	for _, rule := range rules {
 		alertingRule := apimodels.AlertingRule{
 			State:       "inactive",