Support offline mode after the first Playground page load (#1643)

A set of bugfixes and enhancements for [PR 1535 – Cache Playground
assets to enable offline
support](#1535):

### 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

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Co-authored-by: Bero <berislav.grgicak@gmail.com>