Contribution Material

This MR adds the possibility to upload material for contributions. The uploaded material will also be shown as session material and I implemented some shortcuts for upload and download of the material.

Event organizers must enable the upload for the contribution participants, e.g. at https://hcdc.hereon.de/datahub/events/data-science-symposium-7/edit/

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They must also specify the available licenses for uploads

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Presenting authors (and session conveners or event organizers) find then an upload button next to their card

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and they find it in the sidebar of their contribution

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@viktoria.wichert, @linda.baldewein and @csteger. I will let you know when this feature is available for the data science symposium