Don't import talks from schedule in rooms without video

The schedule importer doesn't seem to know that some rooms have no video coverage.

As the import needs explicitly setting the altname in the talks table, the importer could ignore talks in rooms without an altname.

Note that sooms currently seem to be global (across events). It might be better to have rooms per event, to allow adding more info to the rooms. Changing that probably isn't very difficult, but changing it in a way that's backwards compatible might be a pain