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/v178.go

@@ -0,0 +1,18 @@
+// 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 (
+	"xorm.io/xorm"
+)
+
+func AddLFSMirrorColumns(x *xorm.Engine) error {
+	type Mirror struct {
+		LFS         bool   `xorm:"lfs_enabled NOT NULL DEFAULT false"`
+		LFSEndpoint string `xorm:"lfs_endpoint TEXT"`
+	}
+
+	return x.Sync2(new(Mirror))
+}