Retrieve and cache IIIF manifests via the API

In order to provide consistency when IIIF needs to be generated dynamically -- notably in the case of the preview site, where "previewable" resources, not included in the pre-generated manifests, must be considered -- we are fetching the IIIF manifest via the API, with caching. Note that, if the API returns an error response, the public app will still fall back to the static IIIF file that it can access, if available (otherwise it's a 404).

While it was hoped that we would be able to provide manifests dynamically in production (and hence cease pre-generating manifests), more work would need to be done to achieve sufficient efficiency in the process to satisfy stakeholders.