Set up a store for fetched data

[jayperryworks]

• display a preloader/progress bar/something as next page fetches data
• hoard fetched/prefetched data into local store so we don't have to keep querying it on page reload

[jayperryworks]

Apparently you can't access the store in Sapper during preload because nothing has instantiated yet - and I'm also not sure to what extent a store is actually needed, as data is already being cached by the service worker, and injected inline during the export script. It doesn't seem worth the effort right now to figure out a workaround for this, esp since visitors are unlikely the visit the same page twice all that often.