Set up automated backups with public key encryption

[ravicious]

We need to have a solution which lets us back up the database without other people in Lunar having access to the backups.

Now, the tool we use for that (backup) supports GPG encryption, but our Ansible role doesn't support appropriate options to set it up.

We'll need to make it so that only the organizers of Rails Girls have the private key required for decrypting the backup files.