Alerting: Add endpoint to revert to a previous alertmanager configuration (#65751)

* Alerting: Add endpoint to revert to a previous alertmanager configuration

This endpoint is meant to be used in conjunction with /api/alertmanager/grafana/config/history to
revert to a previously applied alertmanager configuration. This is done by ID instead of raw config
string in order to avoid secure field complications.

pkg/services/ngalert/api/api_alertmanager.go

@@ -228,6 +228,38 @@ func (srv AlertmanagerSrv) RouteGetSilences(c *contextmodel.ReqContext) response
 	return response.JSON(http.StatusOK, gettableSilences)
 }
 
+func (srv AlertmanagerSrv) RoutePostGrafanaAlertingConfigHistoryActivate(c *contextmodel.ReqContext, id string) response.Response {
+	confId, err := strconv.ParseInt(id, 10, 64)
+	if err != nil {
+		return ErrResp(http.StatusBadRequest, err, "failed to parse config id")
+	}
+
+	err = srv.mam.ActivateHistoricalConfiguration(c.Req.Context(), c.OrgID, confId)
+	if err != nil {
+		var unknownReceiverError notifier.UnknownReceiverError
+		if errors.As(err, &unknownReceiverError) {
+			return ErrResp(http.StatusBadRequest, unknownReceiverError, "")
+		}
+		var configRejectedError notifier.AlertmanagerConfigRejectedError
+		if errors.As(err, &configRejectedError) {
+			return ErrResp(http.StatusBadRequest, configRejectedError, "")
+		}
+		if errors.Is(err, store.ErrNoAlertmanagerConfiguration) {
+			return response.Error(http.StatusNotFound, err.Error(), err)
+		}
+		if errors.Is(err, notifier.ErrNoAlertmanagerForOrg) {
+			return response.Error(http.StatusNotFound, err.Error(), err)
+		}
+		if errors.Is(err, notifier.ErrAlertmanagerNotReady) {
+			return response.Error(http.StatusConflict, err.Error(), err)
+		}
+
+		return ErrResp(http.StatusInternalServerError, err, "")
+	}
+
+	return response.JSON(http.StatusAccepted, util.DynMap{"message": "configuration activated"})
+}
+
 func (srv AlertmanagerSrv) RoutePostAlertingConfig(c *contextmodel.ReqContext, body apimodels.PostableUserConfig) response.Response {
 	currentConfig, err := srv.mam.GetAlertmanagerConfiguration(c.Req.Context(), c.OrgID)
 	// If a config is present and valid we proceed with the guard, otherwise we