Include original supplemental filenames in iiif manifest

[masaball]

This PR adds an entry to the label field for caption and transcript files so that the original filename can be serialized into the manifest. We serialize the user-defined label into the manifest first, using the language as its key. Then we serialize the original filename without a key.

This will allow us to differentiate the names to facilitate better file handling within the Ramp file downloader.

[cjcolvar]

The code looks good!
I'm a little concerned about putting two different values that don't have equivalent semantics in the same IIIF field, but we can think about that in another issue.