Option to customise thumbnail display

[lukew-cogapp]

Hello, enhancement ideas for handling thumbnails:

• Add option to render thumbnails as icons
  • Use case is media with not-very-interesting thumbnails that will all look the same at 200px
  • Benefit is taking up less space and is slightly related to https://github.com/samvera-labs/clover-iiif/issues/86 for performance reasons
• Add option to not render thumbnails at all
  • Use case is media with a few pages only, we may decide it's not worth displaying the thumbnails at all
  • In this instance the left/right and page indicator could still be useful, but their position may need to shift
  • In this instance the page search is not useful