[keyserver] Script to scp backups over from a server

Summary:
The Docker host can generate backups, but the backups are useless if they're stored in the same place.

This script is intended to be run on a different machine. It will scp the latest backup from a source folder and make sure disk usage doesn't exceed some limit in the destination.

Test Plan:

• I copy-pasted this script to my personal server, where I ran it to scp files over from my keyserver on AWS: bash/scp-backup.sh backups backups
• I created dummy backups in the destination to fill up disk space
• I made sure that when disk space was exceeded, the oldest backups got deleted first
• I made sure that the cleanup never deleted all backups (there was always one left, even if it's bigger than the limit)

Reviewers: atul, yayabosh

Reviewed By: yayabosh

Subscribers: palys-swm, Adrian

Differential Revision: https://phab.comm.dev/D4288