Refactor ladders.js for ES6 class syntax
Refactor ladders.js for ES6 class syntax

One day, we'll convert all our files, but for now I'm just doing
this one as an example. I chose this one specifically because
it's reasonably small and pretty modern.

While I'm at it, I also improved the commenting a bit.

EienSeiryuu/Pokemon-Showdown,CreaturePhil/Showdown-Boilerplate,xfix/Pokemon-Showdown,HoeenCoder/SpacialGaze,danpantry/Pokemon-Showdown,AWailOfATail/Pokemon-Showdown,kuroscafe/Showdown-Boilerplate,cadaeic/Pokemon-Showdown,BlazingAura/Showdown-Boilerplate,Ecuacion/Pokemon-Showdown,Irraquated/Pokemon-Showdown,ZestOfLife/PSserver,TheManwithskills/COC,Lord-Haji/SpacialGaze,KewlStatics/Pokeindia-Codes,Wando94/Spite,xfix/Pokemon-Showdown,QuiteQuiet/Pokemon-Showdown,zek7rom/SpacialGaze,sirDonovan/Pokemon-Showdown,xCrystal/Pokemon-Showdown,Lord-Haji/SpacialGaze,SkyeTheFemaleSylveon/Pokemon-Showdown,KewlStatics/Pokeindia-Codes,jd4564/Pokemon-Showdown,Vacate/Pokemon-Showdown,Irraquated/Pokemon-Showdown,CreaturePhil/Showdown-Boilerplate,ZestOfLife/FestiveLife,Pikachuun/Pokemon-Showdown,KewlStatics/Shit,svivian/Pokemon-Showdown,psnsVGC/Wish-Server,aakashrajput/Pokemon-Showdown,QuiteQuiet/Pokemon-Showdown,Elveman/RPCShowdownServer,TbirdClanWish/Pokemon-Showdown,Pikachuun/Pokemon-Showdown-SMCMDM,aakashrajput/Pokemon-Showdown,CharizardtheFireMage/Showdown-Boilerplate,Zarel/Pokemon-Showdown,Lord-Haji/SpacialGaze,azum4roll/Pokemon-Showdown,Alpha-Devs/OmegaRuby-Server,KewlStatics/Shit,Flareninja/Showdown-Boilerplate,DesoGit/Tsunami_PS,Flareninja/Showdown-Boilerplate,DesoGit/TsunamiPS,svivian/Pokemon-Showdown,kotarou3/Krister-Pokemon-Showdown,PS-Spectral/Spectral,svivian/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,xfix/Pokemon-Showdown,Syurra/Pokemon-Showdown,AbsolitoSweep/Absol-server,DesoGit/Tsunami_PS,Zarel/Pokemon-Showdown,Hidden-Mia/Roleplay-PS,ShowdownHelper/Saffron,Alpha-Devs/Alpha-Server,Lauc1an/Perulink-Showdown,KewlStatics/Showdown-Template,danpantry/Pokemon-Showdown,Irraquated/Pokemon-Showdown,Pikachuun/Pokemon-Showdown,Sora-League/Sora,Hidden-Mia/Roleplay-PS,urkerab/Pokemon-Showdown,AustinXII/Pokemon-Showdown,xfix/Pokemon-Showdown,panpawn/Gold-Server,Enigami/Pokemon-Showdown,Firestatics/omega,Wando94/Spite,ehk12/coregit,panpawn/Gold-Server,kuroscafe/Showdown-Boilerplate,SkyeTheFemaleSylveon/Pokemon-Showdown,jumbowhales/Pokemon-Showdown,AustinXII/Pokemon-Showdown,Zarel/Pokemon-Showdown,TheManwithskills/Creature-phil-boiler,DarkSuicune/Pokemon-Showdown,Mystifi/Exiled,Guernouille/Pokemon-Showdown,DesoGit/TsunamiPS,Enigami/Pokemon-Showdown,xfix/Pokemon-Showdown,Kokonoe-san/Glacia-PS,SolarisFox/Pokemon-Showdown,TheFenderStory/Pokemon-Showdown,AWailOfATail/Pokemon-Showdown,kuroscafe/Paradox,CreaturePhil/Showdown-Boilerplate,jumbowhales/Pokemon-Showdown,Mystifi/Exiled,panpawn/Pokemon-Showdown,arkanox/MistShowdown,psnsVGC/Wish-Server,xCrystal/Pokemon-Showdown,ShowdownHelper/Saffron,Firestatics/omega,Bryan-0/Pokemon-Showdown,Volcos/SpacialGaze,AnaRitaTorres/Pokemon-Showdown,PS-Spectral/Spectral,Elveman/RPCShowdownServer,Git-Worm/City-PS,zek7rom/SpacialGaze,Alpha-Devs/Alpha-Server,Sora-League/Sora,Pikachuun/Pokemon-Showdown-SMCMDM,svivian/Pokemon-Showdown,Enigami/Pokemon-Showdown,svivian/Pokemon-Showdown,AustinXII/Pokemon-Showdown,Vacate/Pokemon-Showdown,Guernouille/Pokemon-Showdown,KewlStatics/Alliance,Bryan-0/Pokemon-Showdown,KewlStatics/Alliance,QuiteQuiet/Pokemon-Showdown,Sora-League/Sora,DarkSuicune/Pokemon-Showdown,cadaeic/Pokemon-Showdown,TheManwithskills/COC,Volcos/SpacialGaze,aakashrajput/Pokemon-Showdown,BlazingAura/Showdown-Boilerplate,Ecuacion/Pokemon-Showdown,KewlStatics/Showdown-Template,panpawn/Gold-Server,Mystifi/Exiled,Kokonoe-san/Glacia-PS,ZestOfLife/FestiveLife,EienSeiryuu/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,kotarou3/Krister-Pokemon-Showdown,HoeenCoder/SpacialGaze,ehk12/coregit,Enigami/Pokemon-Showdown,jd4564/Pokemon-Showdown,xCrystal/Pokemon-Showdown,Syurra/Pokemon-Showdown,Alpha-Devs/OmegaRuby-Server,ZestOfLife/PSserver,TheManwithskills/Creature-phil-boiler,panpawn/Pokemon-Showdown,urkerab/Pokemon-Showdown,TbirdClanWish/Pokemon-Showdown,HoeenCoder/SpacialGaze,urkerab/Pokemon-Showdown,Git-Worm/City-PS,azum4roll/Pokemon-Showdown,DesoGit/Tsunami_PS,AWailOfATail/Pokemon-Showdown,ShowdownHelper/Saffron,jumbowhales/Pokemon-Showdown,kuroscafe/Paradox,AnaRitaTorres/Pokemon-Showdown,HoeenCoder/SpacialGaze,CharizardtheFireMage/Showdown-Boilerplate,AbsolitoSweep/Absol-server,SolarisFox/Pokemon-Showdown,Lauc1an/Perulink-Showdown,PS-Spectral/Spectral,arkanox/MistShowdown,TheFenderStory/Pokemon-Showdown