add goatcounter analytics

I currently hold 7 days of logs on the webserver, including User-Agent,
referrer, and IP address information. That information gets processed by
goaccess and sent by email. Those reports contain Personnally
Identifiable Information (IP and User Agent, mostly) and so are not
properly anonymized.

Goatcounter is an attempt at fixing that. The data it keeps is somewhat
anonymized, and it is believed (by the author) that this makes it GDPR
compliant.

I use both a Javascript tracker (to get stuff like resolution) and a
pixel tracker (for those who turn off Javascript). Ideally, I would just
parse logs and not bother people with extra queries, but goatcounter
doesn't support parsing logs for now, so that will have to do.

I do not make this decision lightly. I believe goatcounter will be a
*better* system for showing which of my posts are popular than the
current approach, and will therefore be a net win in the privacy of my
users.

See also https://anarc.at/services/analytics/