Split migrations folder (#21549)

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

models/migrations/v1_13/v148.go

@@ -0,0 +1,14 @@
+// Copyright 2020 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_13 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func PurgeInvalidDependenciesComments(x *xorm.Engine) error {
+	_, err := x.Exec("DELETE FROM comment WHERE dependent_issue_id != 0 AND dependent_issue_id NOT IN (SELECT id FROM issue)")
+	return err
+}