Split migrations folder (#21549)

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

models/migrations/v1_16/v204.go

@@ -0,0 +1,15 @@
+// Copyright 2021 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_16 //nolint
+
+import "xorm.io/xorm"
+
+func AddSSHKeyIsVerified(x *xorm.Engine) error {
+	type PublicKey struct {
+		Verified bool `xorm:"NOT NULL DEFAULT false"`
+	}
+
+	return x.Sync2(new(PublicKey))
+}