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/v207.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 AddWebAuthnCred(x *xorm.Engine) error {
+	// NO-OP Don't migrate here - let v210 do this.
+
+	return nil
+}