Add support for pairwise judging

[ikhatri]

Lots of hackathons use MIT's Gavel for judging. Their pairwise algorithm is pretty good and we should support it as a judging option within Dashboard. See: https://www.anishathalye.com/2015/11/09/implementing-a-scalable-judging-system/ for details, it's gonna be a bit of a mathy implementation and you'll have to read the paper for details :)

The Judging algorithm options should be (for now): {classic_paper, classic_online, pairwise}