Previously when the deduplicate_book_data script tried to merge an
edition that was on a shelf or in a list then it would fail because when
the canonical book was added to the shelf or the list then it wouldn’t
set the extra fields of the linking table for the “through” model of the
field. These would end up defaulting to NULL, but that is not valid for
some of the fields in ShelfItem and ListItem so postgres wouldn’t accept
it.
To fix that, this patch makes it skip updating fields that have a
non-autogenerated linking table. The linking table would appear as a
separate model anyway so the book will be moved via that instead.
Fixes: #2817
