Link existing contributors and organizations through the API

Linking an existing author, editor or organization to a publication through the API did not always pick the record you meant: depending on what you sent, a duplicate was created or the entry was ignored. That is fixed. You now link an author or editor to the existing entry of your account by id or name, and an organization by id. An id that does not exist in your account is answered with a 404 instead of quietly saving something else.

An authority is also linked at most once per role: if a request lists the same person several times as author or editor, the publication shows them once. This applies to API v2 and v3.