Data (teams and prefs) are now always stored in localStorage in the
`https://play.pokemonshowdown.com` origin. Data storage at a low
level has been mostly rewritten for this, and moved to storage.js.
Tools.prefs has been renamed Storage.prefs, although the API
Tools.prefs(prefName) is still available, and recommended for
battle.js (because storage.js isn't available in replays). All
other pref functionality, such as setting prefs and saving prefs,
should only be used in Storage.prefs.
If the browser blocks third-party cookies in a detectable way,
data is instead stored in the local origin.
As a final safety measure, prefs/teams are mirrored in the local origin
in case third-party cookies are blocked in an undetectable way.
This refactor also replaces the 'init:loadprefs' and 'init:loadteams'
events with a new load tracking system that's basically a simplified
implementation of Promises, documented in the comments for
Tools.makeLoadTracker().
