Add "scrubbed backups" capability

Adds a --scrubbed-backup flag for backup generation for development use.  This generates backups with all user-identifying and authentication information (email addresses, passwords, password reset nonces) removed.  Email addresses are replaced with a non-existent address, and all user accounts in the backup are given the password "test".

You can use a scrubbed backup to restore into a database that you can interact with just like the main Hackage database.  This is a more effective means of making a development test database than mirroring individual packages from the main Hackage server since it reproduces all package download statistics from the main server, along with all user accounts and other information from the main server.