abstract out react-route requirement for Asset{List,Form}

[rjw57]

Instead of using directly inside Asset{List,Form}, the "react-y" way is probably to have an onNavigateToAsset prop or similar which is called when the user wants to navigate to an asset. (E.g. by clicking on the item in the list.) This can navigate the browser in the application but can simply set a flag in test to check it was called.