[jed4] Link checker

[JazParkyn]

Each day the URLs (download, documentation, support, demo, license, project) of every extension should be checked to see if they still work. If they don’t work, an email should go out to the user to get it working again. After 24 hour grace period, the listing will be unpublished (with a new email sent to the developer). After the user updates the listing (or during the regular daily check) and the URLs work again, the listing should be republished (with a new email to sent to the developer).