Add Crossmark metadata to chapters in Crossref DOI deposit export

[brendan-oconnell]

Crossmark metadata will soon be included in Thoth export for Crossref DOI deposit for non-Chapter Works (e.g. MONOGRAPH, EDITED BOOK, etc.); see #603

Although Crossref allows Chapter-level Crossmark metadata (see Crossref Schema section on Element content_item), we have chosen not to currently implement this, due to lack of potential use cases in Thoth for the time being. Crossmark metadata at the book level makes sense: publishers will be able to use the new SUPERSEDED work_status to denote an older edition of a book, for example, link to the new edition as a Related Work in Thoth that REPLACES it, and this metadata will be reflected in Crossmark and can used in the Crossmark button in PDFs.

For now, this is the major expected use case for Crossmark (along with works that have been WITHDRAWN); we do not anticipate new editions of individual chapters to be published without the re-publication of the entire book, so exporting Crossmark metadata on individual chapters is unnecessary. However, if a use case arises, this can be implemented in the future.