updated migration script to allow notifications icons to go grey

Same script as in https://github.com/HabitRPG/habitrpg/pull/3907 but updated to use a node.js stream-based migration as described in https://github.com/HabitRPG/habitrpg/pull/3907#issuecomment-53522251

This script modifies all users' newMessages object to remove all guilds with the "undefined" key and all guilds with the false value for new messages.

@lefnire, this definitely needs you to check it before it's used. I was making it up as I went along. :) Note the "XXX" comment where you have to choose a database.

If this script is close to what you're after, I'll likely use it as a template for future scripts, so IFF you have time, I'd appreciate any comments you want to make.