use id as key for chapters to avoid dnd problems

app/components/BookBuilder/Division.jsx

@@ -142,7 +142,7 @@ export class Division extends React.Component {
           chapter={c}
           id={c.id}
           ink={ink}
-          key={c.index}
+          key={c.id}
           move={_onMove}
           no={i}
           outerContainer={outerContainer}