Fix for MQC 9231, involving three new checks in the app.  First, we had not previously expected an assigned book to have quantity zero; now we check and if it's zero we ignore it.  Second, whereas we previously disallowed more than one contentMetadataItem per book assignment, we now allow it but arbitrarily choose just one for display.  Third, when populating a bookshelf with contentMetadataItems we now check that a given isbn (of a given DRM type) is not added more than once.