Rename shelf_book table's book_id to item_id
Rename ShelfBook model to ShelfItem
Rename Shelf's book_ids to item_ids
Add source and item_type to shelf_item
Refactor books controller and modules for use by shelves
Aggregate book and dpla items in showy actions
Adjustments for merging both book and dpla types in shelf item json
Final tweaks for dpla bookmarklet
Fix template error when creating shelves