Split migrations folder (#21549)

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

models/migrations/v150.go

@@ -1,39 +0,0 @@
-// Copyright 2020 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 migrations
-
-import (
-	"code.gitea.io/gitea/modules/timeutil"
-
-	"xorm.io/xorm"
-)
-
-func addPrimaryKeyToRepoTopic(x *xorm.Engine) error {
-	// Topic represents a topic of repositories
-	type Topic struct {
-		ID          int64  `xorm:"pk autoincr"`
-		Name        string `xorm:"UNIQUE VARCHAR(25)"`
-		RepoCount   int
-		CreatedUnix timeutil.TimeStamp `xorm:"INDEX created"`
-		UpdatedUnix timeutil.TimeStamp `xorm:"INDEX updated"`
-	}
-
-	// RepoTopic represents associated repositories and topics
-	type RepoTopic struct {
-		RepoID  int64 `xorm:"pk"`
-		TopicID int64 `xorm:"pk"`
-	}
-
-	sess := x.NewSession()
-	defer sess.Close()
-	if err := sess.Begin(); err != nil {
-		return err
-	}
-
-	recreateTable(sess, &Topic{})
-	recreateTable(sess, &RepoTopic{})
-
-	return sess.Commit()
-}