Database backup: implement private/public backups

[FedericoCeratto]

Add support for backing up database tables to private and public S3 buckets as needed in https://github.com/ooni/backend/blob/master/analysis/ooni_db_backup.py

[FedericoCeratto]

This is related to #191 - publishing the fastpath and JSONL tables provides users with another way to access measurement data.

They also provide indexing of the files on the S3 data bucket. This could be used to selectively download and reprocess only the most interesting measurement.

[hellais]

We will probably not end up doing this as this should be the job of ooni/data.