Move migrations into services and base into modules/migration (#17663)

* Move migrtions into services and base into modules/migration

* Fix imports

* Fix lint

modules/migration/repo.go

@@ -0,0 +1,18 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Copyright 2018 Jonas Franz. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migration
+
+// Repository defines a standard repository information
+type Repository struct {
+	Name          string
+	Owner         string
+	IsPrivate     bool `yaml:"is_private"`
+	IsMirror      bool `yaml:"is_mirror"`
+	Description   string
+	CloneURL      string `yaml:"clone_url"`
+	OriginalURL   string `yaml:"original_url"`
+	DefaultBranch string
+}