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/migrations.go

@@ -0,0 +1,22 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package setting
+
+var (
+	// Migrations settings
+	Migrations = struct {
+		MaxAttempts  int
+		RetryBackoff int
+	}{
+		MaxAttempts:  3,
+		RetryBackoff: 3,
+	}
+)
+
+func newMigrationsService() {
+	sec := Cfg.Section("migrations")
+	Migrations.MaxAttempts = sec.Key("MAX_ATTEMPTS").MustInt(Migrations.MaxAttempts)
+	Migrations.RetryBackoff = sec.Key("RETRY_BACKOFF").MustInt(Migrations.RetryBackoff)
+}