Make schedule available in frab XML format

[zeltophil]

Many people already use apps like Giggity that display event schedules. Giggity understands various XML formats, for example frab XML. Using this format might also make it easier when using part of the VOC infrastructure for publishing recordings.

[tykling]

frab XSD (xml schema): https://raw.githubusercontent.com/frab/schedule.xml/master/schema/schedule.xml.xsd

pretalx implementation: https://github.com/pretalx/pretalx/blob/d44f67f0452fe0fdf7336b694790e9604381cc48/src/pretalx/schedule/exporters.py#L90-L105

[tykling]

A bit of background: The frab schedule.xml repo has examples of frab event XML from three different generators here https://github.com/frab/schedule.xml/tree/master/generator

When validated with the XSD here https://raw.githubusercontent.com/frab/schedule.xml/master/schema/schedule.xml.xsd we get the following results:

• XML generated by frab: https://raw.githubusercontent.com/frab/schedule.xml/master/generator/frab/camp2019.xml - doesn't validate Reason: Unexpected child with tag 'generator' at position 1. Tag 'version' expected.
• XML generated by pentabarf: https://raw.githubusercontent.com/frab/schedule.xml/master/generator/pentabarf/fosdem2019.xml doesn't validate Reason: missing required attribute: 'guid'
• XML generated by pretalx: https://raw.githubusercontent.com/frab/schedule.xml/master/generator/pretalx/mrmcd2019.xml validates ok! :+1:

[tykling]

https://wwwstaging.bornhack.org/bornhack-2019/program/frab.xml