HYC-1354 - Fallback Thumbnails

[davidcam-src]

https://unclibrary.atlassian.net/browse/HYC-1354

• Issues with default thumbnails showing up for works with existing filesets after deleting the first is caused by the representative_id field not updating after the first file attached is deleted
• Encountered some issues trying to update the field during the delete operation, made changes to some presenters and views instead

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• Implemented override for retrieve_values in Blacklight's thumbnail presenter to substitute a work's thumbnail path with the thumbnail path for the first associated fileset if it's the default
• Changed representative media partial to rely on a custom function fetch_primary_fileset_id instead of representative_id so the first fileset id is substituted if it's nil
• Use fetch_primary_fileset_id in place of representative_id forrepresentative_presenter in the work_show_presenter
• Decrease the amount of filesets needed to render the File Manager button on work show pages from 2 to 1
• Change load to require_dependency to lessen frequency of errors in local environments caused by loading the same override multiple times