Think about how to improve offline workflow

Over on [twitter somebody is asking about using Slingcode offline](https://twitter.com/Lucas18835766/status/1276474261356580864).

> If slingcode had a service worker, would that allow it to be used on iOs without an internet connection?

> Safari. The PWA on the homescreen now loads without connection - but not always. Running a demoapp navigates to http://slingcode.net - so that doesn’t. Will try a new app.

> I can create a new app. Running it in a new tab doesn’t work offline because it navigates to http://slingcode.net. Running it next to the code works halfway: it shows something, Loading..., and after loading I get plain white contents - not as expected.

> Same white contents as when trying it when online in a new tab - so that doesn’t work either (?).