Show sites with items that have not been updated in the last n days

[pdestefanis]

While a site may be sending updates, they may not include all the items the site is tracking.
Use another color to indicate sites that has not receive reports for one of its items in the last X days.
The logic to detect one of this sites is "Site Alpha has received at least one report for item A, but no more reports for that item in the last X days".
If a site never received a report for item B, item B will not trigger this alert

In the Options menu:
Mark sites which have not sent reports for every item they track? (Yes/No)
Number of days without reports (three digits max)
Mark site with color (color selector)