... pulled out debug.njk into own component because I will be using this to debug merging of page-index.njk with page-list.njk in future commits.

... this stops us from having issues with Nunjucks and async promise resolving within templates.

Instead of the template waiting for the result of the filter function, the function instead returns what the image src will be and enqueues processing the screenshot which will happen during the `eleventy.after` lifecycle hook.

…ex page.

... this completes #385

... 11ty processes global data files found in the `_data` directory first, which makes the data available to collections. Not sure *why* I was loading it via the 11tydata but it meant that for books the lists template was unable to count how many items it contained.

With this change it can now.

…out' into feature/379-385-list-default-layout