Add schedule export support for pentabarf

This export can be used by the Engelsystem to import all talks and
keynotes for volunteer management (video, session chair, ...)

To export run:

    $ python manage.py schedule_pentabarf_export > /tmp/schedule.xcal

or for a nicely formatted xcal file:

    $ python manage.py schedule_pentabarf_export | xmllint --format - > /tmp/schedule.xcal