Add frab xml export

[Kunsi]

To make it easier for VOC to use the schedule for events, a XML export is needed, because some VOC tools only support XML, not JSON.

The XML definition can be found here: https://github.com/voc/schedule/tree/master/validator/xsd

[m4schini]

What does the xml enable that the json does not?

[Kunsi]

The tracker only takes XML imports. That way we can just import the schedule, without needing to inout all the details manually.

[m4schini]

Currently timekeeper does not keep track of the information relevant for this export

Needed Info:

• track members of groups/presentations ( Option to add participants (and other roles) to timeslot #8 )

Plan is to first add the relevant data and then implement the export/extend the json export

[Kunsi]

I think people is the only thing we're missing currently