Unlike after-dirvish, before-dirvish is not run by /etc/dirvish/dirvish-cronjob
because it does preparation tasks.  Additionally, it doesn't need to access the
mounted filesystem.

Multiple invocations from separate cron jobs are required.

One cron job should run at 3:45pm Mon-Fri.  It runs
  before-dirvish --req-drive
which, if the consecutive backup count for the current drive is greater than
<swap_backup_count>, e-mails the staff address to request that the least
recently used drive be brought in tomorrow (or Mon if today is Fri).  The
drive's UUID is recorded as <drive_to_swap>.

Another cron job should run at 8am Mon-Fri.  It runs
  before-dirvish --req-swap
which, if <drive_to_swap> is set, e-mails the staff address to request that
<drive_to_swap> be connected and the current drive be taken home.

Another cron job should run at 3pm Mon-Fri.  It runs
  before-dirvish --remind-swap
which acts like --req-swap except that it uses a sterner subject line.
In addition, it should do nothing if <drive_to_swap> is connected.

TO-DO:
  + request a specific drive (optionally interacts with the user)
  + track drive locations (could this script even do this?)