Add xml export whether livestream exists

[johanvdw]

There is no way to find out easily if a livestream exists (or recordings should be created) from the xml.

From chat:
FOSDEM/pretalx-integration#85 -- is the fact that those are not video talks also shown in the XML dump somehow? If so, where?

I don't think it is
But they should also not be in your video export.
If needed, we could probably add either the track type or a field has_video to the track, similar to how this slug was added: https://github.com/FOSDEM/website/pull/282/files
or per talk?
Or another way would be publishing the video link, so it is also available for apps.