Auto-restrict commenting N days after issue close

We often get new comments on super-old issues with users encountering similar problems. They comment on the old thread and either someone happens to notice it or not, and it gets handled or not.

It would be nice as an option of oss-bot to have it automatically restrict comments N days after an issue is closed and post a message something like:

> This issue has been marked as closed and is now closed for comments. If you are experiencing a similar issue, please file it as a new report with all relevant details.