Implement background task to download api data

[toddjordan]

Since the API data will be kept in separate files, spawn a background thread to download them and populate the data store (indexedDB) while the user is browsing the site.

On initial load the client will fetch a manifest file containing the list of files for the current project/version.
This should kick off a worker that iterates the list of files, fetching each and storing to the local db.

[toddjordan]

closing in favor of #134 and #141