Re-add full ics export


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