Support offline mode after the first Playground page load

[adamziel]

A set of bugfixes and enhancements for PR 1535 – Cache Playground assets to enable offline support:

Offline mode after a single page load

Without this PR:

When the service worker is first registered, it does not yet handle the network requests triggered by the page that registered it. Files fetched on the initial Playground load, like wordpress-static.zip, are not cached. This means the offline mode won't work yet. You need to do a full page reload, wait for another round of fetch()-es, and only then you get cached assets and a functional offline mode.

With this PR:

Service Worker calls self.clients.claim() to start controlling the Playground page when the service worker is initially registered. Static assets are cached and offline mode works after the very first page load.

Backfill static assets before the first paint

• If wordpress-static.zip is cached, it will be extracted during Playground boot.
• If it's not cached, we'll boot and render Playground first, and only then start downloading and extracting that file.

Other smaller changes

• Simplified OfflineModeCache, used class methods (function myFn(){} instead of instance methods (myFn = () => {})

cc @bgrgicak