Move the caching mechanism (powered by SuperRepo) to the background service worker

[superKalo]

After the service worker was introduced in #45, we can now move the caching mechanism (powered by SuperRepo) to the background.

This will further optimize the use case when many new tabs are opened. Right now, every new tab creates a separate SuperRepo instance (that counts down until the data is out of date and fires a request to the background service worker to get the latest).

Ideally, this should be refactored to a singleton that lives in the background.

[superKalo]

Tech note: workaround to run the SuperRepo lib with the BROWSER_STORAGE in the service worker:

// Define `window` object with chrome ref, SuperRepo lib expects `window` being available
self.window = { chrome };
importScripts('../lib/index.js'); // SuperRepo lib