Explore hibernation and restoration of Playground instances

[bgrgicak]

With the new site management UI users are able to switch between sites.

Each time you open a site Playground boots and loads the / page.
This is ok when you open the site for the first time, but when you reopen it, it would be great if you could continue from the last state.

To do this we could explore how to hibernate the Playground instance inside the website and restore it.

As a first step we could store instances in memory, but ideally, they would be available even when you refresh.