Split migrations folder (#21549)

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

models/migrations/v1_18/v230.go

@@ -0,0 +1,18 @@
+// Copyright 2022 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_18 // nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+// AddConfidentialColumnToOAuth2ApplicationTable: add ConfidentialClient column, setting existing rows to true
+func AddConfidentialClientColumnToOAuth2ApplicationTable(x *xorm.Engine) error {
+	type OAuth2Application struct {
+		ConfidentialClient bool `xorm:"NOT NULL DEFAULT TRUE"`
+	}
+
+	return x.Sync(new(OAuth2Application))
+}