fix(ui): book metadata form
This error is logged for "processinginstructions" when saving the form: "Collection metadata form show mapper: Requested key processinginstructions does not exist!" It looks like the problem is that the field mapper has a mapping for "processinginstruction" and this should be "processinginstructions".
Also adds a comment noting that "Dropdown" does not expect values to be "uuid" type.
Related to #1598 (closed)