Move issue change content from models to service (#8711)

* Move issue change content from models to service

* fix lint

services/issue/content.go

@@ -0,0 +1,23 @@
+// 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 issue
+
+import (
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/notification"
+)
+
+// ChangeContent changes issue content, as the given user.
+func ChangeContent(issue *models.Issue, doer *models.User, content string) (err error) {
+	oldContent := issue.Content
+
+	if err := issue.ChangeContent(doer, content); err != nil {
+		return err
+	}
+
+	notification.NotifyIssueChangeContent(doer, issue, oldContent)
+
+	return nil
+}