Add read support for xCal events

This is not exposed as public API (yet) on its own as it's still not entirely complete. It's only accessible via FileStorage, allowing that to read more content than before.

In particular this only supports reading and supports events so far, but for those both the final RFC 6321 variant as well as an older draft standard.

This is motivated by Pretalx having removed iCal export support, something Kongress relied on. Pretalx still has xCal export though (using the older RFC draft).

Getting this into 6.6 would be ideal, as there's several conferences next month (SotM, Matrix) which wont work in Kongress without this.