Split migrations folder (#21549)

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

models/migrations/v1_8/v80.go

@@ -0,0 +1,17 @@
+// Copyright 2019 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_8 // nolint
+
+import "xorm.io/xorm"
+
+func AddIsLockedToIssues(x *xorm.Engine) error {
+	// Issue see models/issue.go
+	type Issue struct {
+		ID       int64 `xorm:"pk autoincr"`
+		IsLocked bool  `xorm:"NOT NULL DEFAULT false"`
+	}
+
+	return x.Sync2(new(Issue))
+}