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/v191.go

@@ -0,0 +1,29 @@
+// 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 (
+	"code.gitea.io/gitea/modules/setting"
+
+	"xorm.io/xorm"
+)
+
+func AlterIssueAndCommentTextFieldsToLongText(x *xorm.Engine) error {
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	if setting.Database.UseMySQL {
+		if _, err := sess.Exec("ALTER TABLE `issue` CHANGE `content` `content` LONGTEXT"); err != nil {
+			return err
+		}
+		if _, err := sess.Exec("ALTER TABLE `comment` CHANGE `content` `content` LONGTEXT, CHANGE `patch` `patch` LONGTEXT"); err != nil {
+			return err
+		}
+	}
+	return sess.Commit()
+}