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/v182.go

@@ -0,0 +1,42 @@
+// 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 AddIssueResourceIndexTable(x *xorm.Engine) error {
+	type ResourceIndex struct {
+		GroupID  int64 `xorm:"pk"`
+		MaxIndex int64 `xorm:"index"`
+	}
+
+	sess := x.NewSession()
+	defer sess.Close()
+
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	if err := sess.Table("issue_index").Sync2(new(ResourceIndex)); err != nil {
+		return err
+	}
+
+	// Remove data we're goint to rebuild
+	if _, err := sess.Table("issue_index").Where("1=1").Delete(&ResourceIndex{}); err != nil {
+		return err
+	}
+
+	// Create current data for all repositories with issues and PRs
+	if _, err := sess.Exec("INSERT INTO issue_index (group_id, max_index) " +
+		"SELECT max_data.repo_id, max_data.max_index " +
+		"FROM ( SELECT issue.repo_id AS repo_id, max(issue.`index`) AS max_index " +
+		"FROM issue GROUP BY issue.repo_id) AS max_data"); err != nil {
+		return err
+	}
+
+	return sess.Commit()
+}