When JavaScript is disabled in the browser, we use the Matomo Image Tracker to
gather analytics data. In this case, first party cookies are also disabled and
Matomo is not able to differentiate between different visitors very well. The
tracking ID is intended to provide the same function as the visitor ID that is
usually provided by Matomo when JS is enabled.

The user ID is supposed to identify a registered user of a site, but we are
actually using it to identify a visitor. This change generates a visitor ID in a
similar way to the Matomo JS client, for the particular case where the visitor
does not have a JS-enabled browser. The user ID feature can then be implemented
later on without coming into conflict with this visitor ID feature.