Refactor the Elo processing code into EloProcessor.

Refactor the Elo processing code into EloProcessor.

Before, Elo processing was simply a couple of extra methods that mutated
PlayerGameStat and PlayerElo rows, adding them to a session for updating or
inserting later. In order to support an end-of-match game report, we now need
to hold onto to those intermediate values so they can be passed into a
template. To do this I created an EloProcessor class to perform all of the
computation and an EloWIP class to hold onto all of the intermediate values.
EloProcessor can be used to find out what changed during Elo calculations.