there's some trade-offs here (cookies may be discarded by the browser,
query parameters lost if people use a bookmark), but either works fine
for not re-issuing tokens if someone reloads the web site.

This should hopefully be the final (major) part of the restructuring: a
tracker no longer has to know which trip it is on (and indeed it has no
idea for now), instead the server keeps state about which trips are
currently running and will insert incoming pings in a hopefully
reasonable manner, based on their geoposition & time.

There's lots of associated TODO items here (especially there should be
manual overrides for all this logic in the web ui), but that's work for
a future me.

(incidentally, this also adds support for sending all log messages out
via ntfy-sh)

now mostly independent of the gtfs, but still no live-reloading of it.