Suggest Sensible Filenames on Export

[as2388]

Currently when a dataset is exported from Coda, a UUID filename is suggested.

It would be better if the default reflected the imported dataset name somehow e.g. if "nationality.csv" is imported, we could suggest "nationality_coded.csv" for the exported filename instead of e.g. "930772ff-45ac-4c97-8f7c-f6ad95dfc68e"