Refactor printers and publishers as actors

[cmoad]

Even though we have printers and publishers as separate fields in EMu, we should just export those as actors with a role of "printer" or "publisher" in the json dumps. This is brought to light when looking at the glossary file that is generated (meta/objects-glossary.md). We could easily just set the role for actor instead of repeating the complete structures for printers and publishers.