Display new 'contributors' block on book home page

[SteelWagstaff]

See https://app.moqups.com/in6bz4aHlT/edit/page/a478e80df

As a book author, I want to display more information about contributors to my project on the home page of the book.

If a book has a 'Contributors' back-matter type and the content of that back matter is empty (meaning that the page displays the auto-generated list), we should add a section between the current primary author and book description block with the heading 'Contributors' which displays the profile pictures of the first five contributors to the book (where available) in this order (editors, authors, contributors, translators, illustrators, reviewers), and a final circle with the number of additional total contributors whose images are not shown. If fewer than five contributors have a profile picture, display the circle with the number in it earlier. The text below the images should read 'Learn more about the creators of this book' and should link to the book's 'Contributors' back-matter page.