Split migrations folder (#21549)

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

models/migrations/v1_19/v231.go

@@ -0,0 +1,19 @@
+// 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_19 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddIndexForHookTask(x *xorm.Engine) error {
+	type HookTask struct {
+		ID     int64  `xorm:"pk autoincr"`
+		HookID int64  `xorm:"index"`
+		UUID   string `xorm:"unique"`
+	}
+
+	return x.Sync(new(HookTask))
+}