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/v190.go

@@ -0,0 +1,24 @@
+// 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 (
+	"fmt"
+
+	"xorm.io/xorm"
+)
+
+func AddAgitFlowPullRequest(x *xorm.Engine) error {
+	type PullRequestFlow int
+
+	type PullRequest struct {
+		Flow PullRequestFlow `xorm:"NOT NULL DEFAULT 0"`
+	}
+
+	if err := x.Sync2(new(PullRequest)); err != nil {
+		return fmt.Errorf("sync2: %w", err)
+	}
+	return nil
+}