[Alerting]: namespace fixes (#34470)

* [Alerting]: forbid viewers for updating rules if viewers can edit

check for CanSave instead of CanEdit

* Clear ngalert tables when deleting the folder

* Apply suggestions from code review

* Log failure to check save permission

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

pkg/tests/api/alerting/api_notification_channel_test.go

@@ -57,7 +57,8 @@ func TestNotificationChannels(t *testing.T) {
 
 	{
 		// Create the namespace we'll save our alerts to.
-		require.NoError(t, createFolder(t, s, 0, "default"))
+		_, err := createFolder(t, s, 0, "default")
+		require.NoError(t, err)
 
 		// Post the alertmanager config.
 		u := fmt.Sprintf("http://grafana:password@%s/api/alertmanager/grafana/config/api/v1/alerts", grafanaListedAddr)