Faceted search of custom metadata

[blms]

• Settings for which metadata fields should be indexed in the django settings
• Read those settings in the indexing code
• Display them on the frontend as facets
• Allow ingest of metadata not on fields; put them in metadata json field
• Render metadata json field as part of metadata in IIIF manifest serialization
• Allow customization of template of metadata pairs on individual volume pages frontend, using a setting (probably not in scope, but I can give guidance; or complete with leftover time)