refactor invenio-files-rest storage factory

[krzysztof]

Most of that code:

invenio-files-rest/invenio_files_rest/storage/pyfs.py

Lines 151 to 169 in 5356812

	assert fileinstance
	fileurl = None
	size = fileinstance.size
	modified = fileinstance.updated
	if fileinstance.uri:
	# Use already existing URL.
	fileurl = fileinstance.uri
	else:
	assert default_location
	# Generate a new URL.
	fileurl = make_path(
	default_location,
	str(fileinstance.id),
	'data',
	current_app.config['FILES_REST_STORAGE_PATH_DIMENSIONS'],
	current_app.config['FILES_REST_STORAGE_PATH_SPLIT_LENGTH'],
	)

Should be factored-out of the storage factory, since it's specific to the way we write FileInstance objects to disk.