[ADD] Added website_sale_attachments module.
[ADD] Added website_sale_attachments_timestamp module.
[IMP] 8.0-website_sale_attachments_timestamp: added tests.
[IMP] 8.0-website_sale_attachments: changed add_attachment to public function and revised unittests to make them more friendly.
Merge remote-tracking branch 'sfl/8.0-website_sale_attachments_timestamp' into 8.0
[IMP] website_sale_attachments_timestamp: fixed the str.format() usage.
[IMP] website_sale_attachments_timestamp: fixed the unittest.