[bug] Custom kwargs in serve_private_file URL pattern

[pandafy]

We have used custom URL kwarg in the serve_private_file URL.

openwisp-firmware-upgrader/openwisp_firmware_upgrader/private_storage/urls.py

Lines 8 to 14 in e166999

	urlpatterns = [
	path(
	urljoin(IMAGE_URL_PATH, '<path:imagefile>'),
	views.firmware_image_download,
	name='serve_private_file',
	),
	]

openwisp-firmware-upgrader/openwisp_firmware_upgrader/private_storage/views.py

Lines 6 to 11 in e166999

	class FirmwareImageDownloadView(PrivateStorageDetailView):
	model = load_model('FirmwareImage')
	model_file_field = 'file'
	slug_field = 'file'
	slug_url_kwarg = 'imagefile'

This breaks the functionality of PrivateS3BotoStorage which uses this URL with path keyword argument. We should change imagefile to path for consistency with private_storage module.

https://github.com/edoburu/django-private-storage/blob/cfe1e62e2458798760fe6f6c9825e2f663d25ac1/private_storage/storage/s3boto3.py#L58-L64