Add retry for migration http/https requests (#9019)

* Add retry for migration http/https requests

* give the more suitable name for retry configuraion items

* fix docs and lint

* Only use retryDownloader when setting > 1

modules/setting/setting.go

@@ -1062,6 +1062,7 @@ func NewServices() {
 	newRegisterMailService()
 	newNotifyMailService()
 	newWebhookService()
+	newMigrationsService()
 	newIndexerService()
 	newTaskService()
 }