Alerting: deactivate an Alertmanager configuration (#36794)

* Alerting: deactivate an Alertmanager configuration

Implement DELETE /api/alertmanager/grafana/config/api/v1/alerts
by storing the default configuration which stops existing cnfiguration
from being in use.

* Apply suggestions from code review

pkg/services/ngalert/api/api.go

@@ -25,6 +25,7 @@ var timeNow = time.Now
 type Alertmanager interface {
 	// Configuration
 	SaveAndApplyConfig(config *apimodels.PostableUserConfig) error
+	SaveAndApplyDefaultConfig() error
 	GetStatus() apimodels.GettableStatus
 
 	// Silences