Import Original and Supporting files separately

This change modifies the import side to assume that the export side knows
which files are originals and which are supporting and exports them as two
distinct columns.

By defaulting the supporting_file_paths argument in
Importer::Base#initialize to nil, we can continue to use existing
exports with the only caveat being everything's assumed original.

We still attempt to parse works from every file we see (originals then
supporting) but we stop as soon as any one is successfull.