Refactor Calculations.LeagueTableFromLeagueGames

[ceddlyburge]

I did this in a hurry when my brain wasn't up to full speed, there is a lot of duplication. It's well tested and well contained though, so not actually a problem, but it would be satisfying to do.

I could use Sandi Metz flocking rules from 99 bottles of oop, and potentially write a blog post about it, and our submit it as an exercise to exercism