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/v95.go

@@ -0,0 +1,20 @@
+// 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 AddCrossReferenceColumns(x *xorm.Engine) error {
+	// Comment see models/comment.go
+	type Comment struct {
+		RefRepoID    int64 `xorm:"index"`
+		RefIssueID   int64 `xorm:"index"`
+		RefCommentID int64 `xorm:"index"`
+		RefAction    int64 `xorm:"SMALLINT"`
+		RefIsPull    bool
+	}
+
+	return x.Sync2(new(Comment))
+}