Split migrations folder (#21549)

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

models/migrations/v1_7/v73.go

@@ -0,0 +1,19 @@
+// Copyright 2018 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_7 // nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddMustChangePassword(x *xorm.Engine) error {
+	// User see models/user.go
+	type User struct {
+		ID                 int64 `xorm:"pk autoincr"`
+		MustChangePassword bool  `xorm:"NOT NULL DEFAULT false"`
+	}
+
+	return x.Sync2(new(User))
+}