Split migrations folder (#21549)

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

models/migrations/v1_12/v129.go

@@ -0,0 +1,17 @@
+// 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_12 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func PurgeUnusedDependencies(x *xorm.Engine) error {
+	if _, err := x.Exec("DELETE FROM issue_dependency WHERE issue_id NOT IN (SELECT id FROM issue)"); err != nil {
+		return err
+	}
+	_, err := x.Exec("DELETE FROM issue_dependency WHERE dependency_id NOT IN (SELECT id FROM issue)")
+	return err
+}