fix: finish ical feed

[SilasPeters]

I recommend reviewing per commit, as this PR includes the reversion of a reversion of a previous PR merge

In #1101 I tried to close #1037, but on staging it did not seem to work fully. After some testing, this should have fixed it!

Aside from several improvements, I solved the issue where datetimes containing just dates - and no implicit information on how to deal with timezones - would be converted to utc. This was an issue, because if 10-10-2010 00:00 would be set a an hour back, it would become 09-10-2010 23:00, whilst we just wanted the date which now moved a day back.

[SilasPeters]

Somehow, it works on my machine but not on staging... Full days start a day too early. Is this a timezone thing?

Edit: the problem seems to lay at caching, for the SAME link works in thunderbird, but not properly in google calendar, and especially because google calendar has out-of-date activity descriptions. Unenrolling from an activity is not reflected in the google calendar, but is in thunderbird.

https://support.google.com/calendar/thread/41447964/why-does-google-calendar-cache-the-calendar-and-how-do-i-stop-it?hl=en

Checking how others do it...

Update: https://www.ryadel.com/en/google-calendar-force-update-refresh-subscribed-calendar-ics/
Perhaps we should update robots.txt?

Update: after exclusively telling Google to sync it, (and adding it to my mobile app,) it seems to work. Next step is testing how people 'seamlessly' add the UU calendar to their calendar.