Split migrations folder (#21549)

There are too many files in `models/migrations` folder so that I split
them into sub folders.

models/migrations/v1_16/v193.go

@@ -0,0 +1,33 @@
+// Copyright 2021 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 v1_16 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddRepoIDForAttachment(x *xorm.Engine) error {
+	type Attachment struct {
+		ID         int64  `xorm:"pk autoincr"`
+		UUID       string `xorm:"uuid UNIQUE"`
+		RepoID     int64  `xorm:"INDEX"` // this should not be zero
+		IssueID    int64  `xorm:"INDEX"` // maybe zero when creating
+		ReleaseID  int64  `xorm:"INDEX"` // maybe zero when creating
+		UploaderID int64  `xorm:"INDEX DEFAULT 0"`
+	}
+	if err := x.Sync2(new(Attachment)); err != nil {
+		return err
+	}
+
+	if _, err := x.Exec("UPDATE `attachment` set repo_id = (SELECT repo_id FROM `issue` WHERE `issue`.id = `attachment`.issue_id) WHERE `attachment`.issue_id > 0"); err != nil {
+		return err
+	}
+
+	if _, err := x.Exec("UPDATE `attachment` set repo_id = (SELECT repo_id FROM `release` WHERE `release`.id = `attachment`.release_id) WHERE `attachment`.release_id > 0"); err != nil {
+		return err
+	}
+
+	return nil
+}