Split migrations folder (#21549)

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

models/migrations/v1_10/v91.go

@@ -0,0 +1,26 @@
+// Copyright 2019 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_10 //nolint
+
+import "xorm.io/xorm"
+
+func AddIndexOnRepositoryAndComment(x *xorm.Engine) error {
+	type Repository struct {
+		ID      int64 `xorm:"pk autoincr"`
+		OwnerID int64 `xorm:"index"`
+	}
+
+	if err := x.Sync2(new(Repository)); err != nil {
+		return err
+	}
+
+	type Comment struct {
+		ID       int64 `xorm:"pk autoincr"`
+		Type     int   `xorm:"index"`
+		ReviewID int64 `xorm:"index"`
+	}
+
+	return x.Sync2(new(Comment))
+}