Following up on #13363

It would be valuable to be alerted by email, when the tracking of data on a website which used to work, now doesn't track anything.

• email could be sent after 1 week of no data, so that small traffic websites don't get alerted (hopefully each real website at least gets 1 visit per week?)
• If it is on purpose to that data is not tracked anymore (ie. inactive site, closed site), then users can just ignore the email and they won't receive it again. The email should say something like "If the website was closed, or if the Matomo tracking code was removed on purpose, you can ignore this email."
• we'd want to send the email once per website at the most, until any data is tracked again
• If data is tracked again, we can send the same email again after 1 week if no data was tracked then. But this could maybe get annoying if the website is very low traffic and gets eg. 2 visits per month...

The goal is to alert users of possible data loss and make them aware pro-actively (or rather reactively, as at least 1 week of data would be lost already).