The demo application and the book had diverged slightly. This commit
should contain all the updates to the Platformer.elm file so that the
demo follows the book content. And this will allow me to push some
additional updates soon.

Not sure if it's a good idea to alias the Accounts module here since I
think the contexts are separated for a reason. But the idea is to sum
the player's total score whenever a new gameplay record is created.

This works for rendering all the gameplays, but the
http://0.0.0.0:4000/api/gameplays endpoint still needs to be fixed so
that it includes the game_id and player_id.

Got stuck pretty bad here for a while. The good news is it seems like a
good start to be able to query for things like `player.games` and
`player.gameplays` and `game.players` and `game.gameplays`.
The schemas seem okay, but I was having a tough time figuring out how
`cast_assoc`, `build_assoc`, and `put_assoc` work. In the current state,
it looks like players and games are properly linked, but the gameplays
get created without loading the associations properly.