Support for level0 thumbnail services

This is recommended by the IIIF cookbook as the most efficient way to generate thumbnails. 

 * Recipe: [Implementation discussion: Thumbnails on Canvases](https://iiif.io/api/cookbook/recipe/0232-image-thumbnail-canvas/) 
 * [ThumbnailPanel](https://thumbnail-panel.netlify.app/?iiif-content=https://iiif.io/api/cookbook/recipe/0232-image-thumbnail-canvas/manifest-image.json)

Ref #4 