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/v185.go

@@ -0,0 +1,22 @@
+// 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 AddRepoArchiver(x *xorm.Engine) error {
+	// RepoArchiver represents all archivers
+	type RepoArchiver struct {
+		ID          int64 `xorm:"pk autoincr"`
+		RepoID      int64 `xorm:"index unique(s)"`
+		Type        int   `xorm:"unique(s)"`
+		Status      int
+		CommitID    string `xorm:"VARCHAR(40) unique(s)"`
+		CreatedUnix int64  `xorm:"INDEX NOT NULL created"`
+	}
+	return x.Sync2(new(RepoArchiver))
+}