Add features to export/restore environment variables in balenaCloud

Past device environment variables are not saved anywhere by Balena and cannot be restored if accidentally reset (e.g. via PATCH curl command to whole org). Organisations could set up their own backup system using the GET curl commands for getting all device environment variables per device id per fleet to export to a backup file somewhere. But, I think restoring from changes made recently (up to say, last 7 days) would be a useful feature to have.