implement frontend prefill function

This merge request belongs to following backend change: Merge Request for API

I implemented some frontend changes, which are required to upload a second (optional) file. I also implemented the prefill of the formfields through the data from the .json file, if one is uploaded. I also implemented a new backend call based on whether an optional .json file was uploaded or not. And I implemented the handling of different datatypes to ensure a proper prefill of the formfields.

Unfortunately, although I have got a working gitlab runner instance, the pipelines do not complete due to an issue I do not fully understand. @moosmannp is this a known problem?

Signed-off-by: Justin Dex justin.dex@msg-david.de