Split migrations folder (#21549)

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

models/migrations/v1_17/v214.go

@@ -0,0 +1,23 @@
+// 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_17 // nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddAutoMergeTable(x *xorm.Engine) error {
+	type MergeStyle string
+	type PullAutoMerge struct {
+		ID          int64      `xorm:"pk autoincr"`
+		PullID      int64      `xorm:"UNIQUE"`
+		DoerID      int64      `xorm:"NOT NULL"`
+		MergeStyle  MergeStyle `xorm:"varchar(30)"`
+		Message     string     `xorm:"LONGTEXT"`
+		CreatedUnix int64      `xorm:"created"`
+	}
+
+	return x.Sync2(&PullAutoMerge{})
+}