Allow more control over contributor types displayed at the top of the webbook home page + title page in exports

[tw77]

Following pressbooks/ideas#134, we've heard the desire to be able to choose, at the individual book level, which kinds of contributors are shown at the top of the webbook home page and in the title page for exports.

Suggested improvements

1. We’re now always showing Editors in the webbook header (and title page in exports), even when the book has Authors. Could showing Editors here be optional (at least, when the book has Authors)?
2. We're now always showing Translators and/or Illustrators in the webbook header (and title page in exports). Could this be optional?
3. We're now always showing Contributors in the title page in exports. Could this be optional?

Note

An interesting enhancement was suggested in pressbooks/ideas#134: the ability to order / reorder the sequence of these fields (Author, Editor, Translator, etc.). This could be explored here or in a separate issue. The improvements suggested above are more limited in scope.

[MichSpieler]

(link to request in ZD)