backup the swh public website off-gandi

We currently rely on gandi snapshots for both the mysql DB and the wordpress media files. We therefore have a SPOF on gandi hosting. We should copy the backup offsite (e.g., via sftp) and include it in our regular backups.

Migrated from T86 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information