Switch Bookshelves Privacy to db record field (instead of account-level)

Right now there isn't a great way to query bookshelves_books based on a patron's privacy settings (e.g. public v. private) this issue proposes using a new `private` table field to `bookshelves_books` so patrons can tailor which books are private v. public.

Maybe the settings isn't a retroactive fix, it just sets a mode for what value will be applied by default.

For the trending view, we could do a get_many of all people preferences similar to
http://openlibrary.org/query.json?type=/type/object¬ifications.public_readlog=no

### Describe the problem that you'd like solved

### Proposal & Constraints

### Additional context

### Stakeholders