Implement ELO for rankings

[michaeljacobdavis]

Rather than the current ranking system, I propose we use something like https://en.wikipedia.org/wiki/Elo_rating_system that takes into account strength of opponents.

Downsides I can see:

• Doesn't take into account peoples strengths and weaknesses when pairing (two good defensive players that are bad on offense).
• Still might encourage people to play easier matches, but I think it would be better than it is currently.

Opinions?
/cc @clogston

[clogston]

Elo or glicko-2 are what we have available to us. Neither solves for mixed teams. :( Could try to base ratings changes by comparing averages of each team? Haven't thought it through. Definitely worth a shot regardless.