Make thumbnail size configurable

[ggeisler]

The Administration: Appearance page has a section enabling the admin to select the size of thumbnails for the search results views. We need a way for that selection to actually affect the size of the thumbnails.

[cbeer]

To implement this, I think we need to:

• [ ] index the path to the IIIF info.json for the thumbnail
• [ ] add Spotlight::Engine configuration defining a mapping between small/medium/large and image dimensions
• [ ] create a new helper method, maybe iiif_thumbnail_url that implements the necessary #render_thumbnail_tag (https://github.com/projectblacklight/blacklight/blob/0c88d2007bf33e6ef31c4743de75411e6354f02e/app/helpers/blacklight/catalog_helper_behavior.rb#L179) API. It should receive a document, and spit out an image tag using the appropriate size IIIF resource
• [ ] Update https://github.com/sul-dlss/spotlight/blob/master/app/models/spotlight/blacklight_configuration.rb#L75 to, by default, use the new helper.