Fix sender of issue notifications

It is the FROM field in mailer configuration that needs be used,
not the USER field, which is for authentication.

Closes https://github.com/gogits/gogs/issues/3615

models/mail.go

@@ -160,7 +160,7 @@ func composeIssueMessage(issue *Issue, doer *User, tplName base.TplName, tos []s
 	if err != nil {
 		log.Error(3, "HTMLString (%s): %v", tplName, err)
 	}
-	msg := mailer.NewMessageFrom(tos, fmt.Sprintf(`"%s" <%s>`, doer.DisplayName(), setting.MailService.User), subject, content)
+	msg := mailer.NewMessageFrom(tos, fmt.Sprintf(`"%s" <%s>`, doer.DisplayName(), setting.MailService.FromEmail), subject, content)
 	msg.Info = fmt.Sprintf("Subject: %s, %s", subject, info)
 	return msg
 }