Split migrations folder (#21549)

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

models/migrations/v1_16/v198.go

@@ -0,0 +1,33 @@
+// 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_16 //nolint
+
+import (
+	"fmt"
+
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+)
+
+func AddTableIssueContentHistory(x *xorm.Engine) error {
+	type IssueContentHistory struct {
+		ID             int64 `xorm:"pk autoincr"`
+		PosterID       int64
+		IssueID        int64              `xorm:"INDEX"`
+		CommentID      int64              `xorm:"INDEX"`
+		EditedUnix     timeutil.TimeStamp `xorm:"INDEX"`
+		ContentText    string             `xorm:"LONGTEXT"`
+		IsFirstCreated bool
+		IsDeleted      bool
+	}
+
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Sync2(new(IssueContentHistory)); err != nil {
+		return fmt.Errorf("Sync2: %w", err)
+	}
+	return sess.Commit()
+}