Diagnostic to check tracking code is installed on target website

We can add a new server side diagnostic to help users verify and diagnose tracking code issues. The diagnostic would fetch the target site's HTML, find the tracking code and check whether it's ok.

Specific checks could include:

• That it's there and available.
• That it uses https:// if force_ssl = 1 (or doesn't use :// if the target site is using https but for some reason matomo isn't).
  • For this one, might be useful to also show notification to admins if found. Not sure how easy it would be to achieve.
• ???

It could also incorporate the "are there visits?" check that is done when displaying the dashboard. If the tracking code is there, but there are still no visits, we can say so giving the user more information they can use to diagnose & resolve their issue.