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/comment.go

@@ -0,0 +1,20 @@
+// 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"
+
+// Comment is a standard comment information
+type Comment struct {
+	IssueIndex  int64  `yaml:"issue_index"`
+	PosterID    int64  `yaml:"poster_id"`
+	PosterName  string `yaml:"poster_name"`
+	PosterEmail string `yaml:"poster_email"`
+	Created     time.Time
+	Updated     time.Time
+	Content     string
+	Reactions   []*Reaction
+}