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 and html5 do not support conditionally
required fields we shall not supply an option for that.
Local file upload is marked as default for the radio-button mode
add handling for upload-modes in repository file

Currently an implementation of .retrieve, which will perform the
fetching of the remote file, is still missing.