Introduce "year" as (optional) parameter to /load-calendar

The /load-calendar handler now takes the year as an argument. This defaults to the current year, or the next year when called late in the year, i.e. in November or December.

The encoding of the URL pattern used since 2022 has been simplified so that the same pattern can be used independently of the year, as long as data.stadt-zuerich.ch sticks to their own convention. Ideally, this will make it unnecessary to modify this code in the future just to load next year's calendars.

Also, diagnostic messages have been improved a bit when loading a calendar fails.