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/milestone.go

@@ -0,0 +1,19 @@
+// 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
+
+import "time"
+
+// Milestone defines a standard milestone
+type Milestone struct {
+	Title       string
+	Description string
+	Deadline    *time.Time
+	Created     time.Time
+	Updated     *time.Time
+	Closed      *time.Time
+	State       string // open, closed
+}