Simplify our use of sessionStorage by not JSON-encoding everything

Never mind! It was a bit hidden, but it turns out that sessionStorage and localStorage can only store strings. From https://developer.mozilla.org/en-US/docs/Web/API/Web_Storage_API/Using_t...

Storage only supports storing and retrieving strings. If you want to save other data types, you have to convert them to strings. For plain objects and arrays, you can use JSON.stringify().

Nice and neat.