WebHost: Refactor weighted-settings.js (ArchipelagoMW#2318)

* Refactor weighted-settings.js

This moves most of the infrastructure into two classes:

* WeightedSettings covers the settings page as a whole. It tracks the
  user's current settings in local storage as well as the game data
  from the server so they don't need to be manually passed around from
  function to function.

* GameSettings covers the settings for a single game, and provides a
  view of the current settings and the game data just for that game.

* Fix item count updating

Author: nex3