Backup custom derivatives in Fedora

[conorom]

To me it seems that if a user goes to the trouble of uploading a custom thumbnail (or poster) for a FileSet, this should be preserved in Fedora. A suitable association already exists (for a thumbnail):
https://github.com/samvera/hydra-works/blob/d78ed6b9f071834c0aa1baf1202d7c0ff0b55f38/lib/hydra/works/models/concerns/file_set/contained_files.rb#L11

The reason I'd like to do this is so that a backup of the repository would contain everything needed to get it back to a working state. You could argue that if we're confident in the storage redundancy of the file system, then we'll be able to grab tmp/derivatives from somewhere and all will be OK. It does seem neater to have it all together though, and if the video posters have some meaning (to a caption, say) then they really are part of the work itself.

[conorom]

Low priority and blocked, I think, by #1508. We should also parking lot so that we agree this is a good idea. For the record, you can add a thumbnail file to a FileSet like so:
Hydra::Works::AddFileToFileSet.call(FileSet.find('999999999'), File.open(File.join(Rails.root, 'spec', 'fixtures', 'csv', 'shipwreck.jpg')), :thumbnail, update_existing: true, versioning: false)

... and presumably this would also work for another association as soon as you add one :-)