Primary / secondary judges as first-class scheduler concepts

[Xyene]

@quantum5 and I both have machines capable of running backup judges for dmoj.ca at ~all times, but they are not calibrated to be quite the same as what we run in production.

If we had a Primary | Secondary choice in the judge model, we could mark them as Secondary and leave them connected to the prod bridge. The scheduler would then be modified to only schedule to secondary judges if there are no online, non-disabled (after #2005) primary judges.

While we haven't had frequent outages of the current judges, we have had a nonzero number and this would increase the reliability of the platform.