feat: show ical/ICS download link in the event infobox

[erral]

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Closes #226

Following code from volto-light-theme I bring here the link to the ICS/ical download of an event.

[stevepiercy]

Thank you!

[erral]

Here are some examples with the corresponding links:

https://testing.plone.org/news-and-events/events/plone-conferences/plone-conference-2025
https://testing.plone.org/news-and-events/events/sprints/salamina-sprint-2025
https://testing.plone.org/news-and-events/events/plone-tune-up-days/plone-tune-up-aug-2025

I have downloaded the ics/ical files and they are correct.