The URL generated with radius:serve_private_file is not consistent with the rest of the API URLs:

./manage.py show_urls | grep batch | grep api
/api/v1/radius/batch/	openwisp_radius.api.views.BatchView	radius:batch
/api/v1/radius/organization/<slug:slug>/batch/<uuid:pk>/pdf/	openwisp_radius.api.views.DownloadRadiusBatchPdfView	radius:download_rad_batch_pdf
/api/v1/radiusbatch/csv/<path:csvfile>	openwisp_radius.private_storage.views.RadiusBatchCsvDownloadView	radius:serve_private_file

Shouldn't this CSV url be as follows?

/api/v1/radius/organization/<slug:slug>/batch/<uuid:pk>/csv/.

We have an inconsistency here, I will create a new issue for this, another problem is that this URL does not show up in the swagger API docs (/api/v1/docs/), let's find way to show it there too.

This is a follow up of #366 (review).

Further follow up: 0e76462 fixes the inconsistencies, but the URL is not showing up in the API docs, it would be great it we can add it in some way, for consistency, so I will leave this open.