Monitor project downloads and deployment launches

[maximlt]

Thanks to GoatCounter we are able to gather some analytics, all of it being publicly available at https://holoviz.goatcounter.com. This is useful to see which examples are the most visited. However, we don't have much insight about the projects downloaded as ZIP and the deployments launched (the deployment platform doesn't offer that, Lumen Monitor was giving us info about Panel deployments but it is not enabled on the new instance). One option would be to add analytics specifically to the download and deployment links, which seems possible via GoatCounter Events (https://www.goatcounter.com/help/events).

[maximlt]

I just remembered we serve the Panel apps with --rest-session-info --session-history -1 meaning that they expose /rest/session_info (e.g. https://portfolio-optimizer.holoviz-demo.anaconda.com/rest/session_info) with the total of sessions served and the history of sessions served with some details. This was originally intended to be used by Lumen Monitor but could be used by some other system.