* Use per-player ratelimit for /username
* Improve the skin system
- Migrates skin getting over to Mojang. I do indeed understand that the Mojang API is more ratelimited and generally harder to use, it should be noted that it has an almost 0% chance of error. Compare that to Ashcon which, on some days, has a 50% chance of actually recognizing your account exists
- Uses CompletableFutures and a ExecutorService for making requests
- Renames SkinDownloader class to SkinManager class
- Makes SkinManager class static
- Limits the /skin command per-player
