Add code to debug an error [1] triggered during an effect in the
`ThreadList` component.

When the effect is run, an attempt to get a reference to a DOM node
rendered by the component using `getElementById` is failing. This could
be because:

 - The component has been unmounted (I didn't think effects could run
   after unmounting, but I'm not certain)
 - The component has been rendered but the DOM has not yet been attached
   to the document
 - The `visibleThreads` value captured by the effect does not match the
   most recently rendered output.

I couldn't reproduce the issue locally. The debugging code added here
should narrow down which of these is happening.

[1] https://sentry.io/organizations/hypothesis/issues/2554918407/