3577 Fix error when a bookmarked item has been deleted

https://code.google.com/p/otwarchive/issues/detail?id=3577

If a bookmark's bookmarkable item had been deleted, bookmarkable.id was nil and the entire page would give an error. Now if the bookmark's bookmarkable item is deleted, it uses the bookmark.id instead. (At some point in the future, it should be persuaded to *always* use bookmark.id when available, since bookmark.id is unique, while bookmarkable.id can be repeated multiple times on a page. This can cause unexpected behavior, such as forms opening in the wrong place, but it's existing unexpected behavior, so I didn't want to go overboard and rewrite all the things.)