add iri and upload-mode to file-upload form.
add js to toggle between file upload types.
use simple-form input instead of collection-radio.
adjust presence-validation for tempfile/iri

As only one of the two needs to be provided, we had to change the
validations (and add a validation for the remote file iri).
Additionally, as simple-form does not add the required tag to fields
which use a conditional presence validation, we added them for both
fields manually.