PlutoUI showcase no longer working with Binder

Hi

On the page 'PlutoUI' showcase, the option to `run in the cloud with binder' no longer seems to be working. This is also true for other websites using PlutoUI for interactivity. Now, on clicking 'run', it get stuck on 'connecting to backend'. The status tracker displays a spawn error.

Start at the PlutoUI showcase page and its “run in the cloud with Binder” option, then reproduce the failure and inspect the status tracker’s spawn error. Done means the showcase and other PlutoUI interactive pages can start in Binder without remaining stuck on “connecting to backend”.

Written by the indexing model from the issue text.