Add `archive` class

This class creates a command, to be run by the `govuk-backup` user, in order
to remove back-ups created more than 30 days ago.

This works fine:

vagrant@obnode0:~$ sudo crontab -u govuk-backup -l
0 9 * * * find /dev/mapper/backup-data/* -mtime +30 -exec rm {} /\;