Extend the character limit of the superflat custom preset code box

Currently it looks like the character limit for the superflat preset code box is only 1230 characters (which looks like a pretty random number, binary speaking. I don't really get where that number comes from).

That restricts creative freedom, in my opinion, as a slightly more complex preset would require plenty more characters.

I found myself spending 4 hours designing a preset only to find out it won't fit (because it spans 1519 characters). I only dream that one day I will be able to play on my preset.

Thank you for your time, Mojang.