Split migrations folder (#21549)

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

models/migrations/v1_15/v183.go

@@ -0,0 +1,39 @@
+// 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_15 //nolint
+
+import (
+	"fmt"
+	"time"
+
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+)
+
+func CreatePushMirrorTable(x *xorm.Engine) error {
+	type PushMirror struct {
+		ID         int64 `xorm:"pk autoincr"`
+		RepoID     int64 `xorm:"INDEX"`
+		RemoteName string
+
+		Interval       time.Duration
+		CreatedUnix    timeutil.TimeStamp `xorm:"created"`
+		LastUpdateUnix timeutil.TimeStamp `xorm:"INDEX last_update"`
+		LastError      string             `xorm:"text"`
+	}
+
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	if err := sess.Sync2(new(PushMirror)); err != nil {
+		return fmt.Errorf("Sync2: %w", err)
+	}
+
+	return sess.Commit()
+}