Calendar plugin should offer ical export and caldav

[sam (Sam Saffron)]

I think @kelv added some of this back per:

Re-add full ics export

Hi, It would be nice to have the behavior that was removed in #231. Specifically, being able to point an ical client towards events.ics or equivalent to get all events on the site. Additionally, the DESCRIPTION, URL & ORGANIZER fields are missing from the download-calendar.js replacement. The first two are more important to me. It would be even nicer to pull in #169 to add “per-topic” event exports, but that’s extra

https://github.com/discourse/discourse/pull/35143

What is the scope of this request?