User interface with input form

[nichtich]

• [x] upload field for files
• [x] optionally select JSON array elements
• [ ] text field / editor with data to be validated
• [x] select input format and version (less important)

[nichtich]

Requires #27

[nichtich]

See https://codemirror.net/6/examples/bundle/ how to integrate CodeMirror as we likely want syntax-highlighting and error display in the input form (aka editor)