Export iCal calendar from ikiwiki

It would be great for people to be able to import the collective calendar using a widespread format.

The (upstream) calendar plugin does not exporting to iCal. Also, it’s not exactly meant for this kind of things, but could plausibly be extended. If we want to go this way, each event will need to have a dedicated page.
The (third-party) fullcalendar plugin supports exporting to iCal, and builds a calendar from “temporal metadata embedded in pages using some new directives”. This looks closer to what we want. If we want to use it, next thing to do is to try it out, and then either have it included upstream as-is (unclear if upstream will want it, since it duplicates existing functionality) or to merge its improvement into the calendar plugin.

Original created by @BitingBird on 7521 (Redmine)

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