Peaceful mode tweaks

Since Just Blood Carts is centred around killing mobs, it would be harder if mobs didn't spawn.

Therefore, it changes how Peaceful mode works. In Peaceful mode, mobs won't disappear, but they won't be able to hurt the player. Nothing else can hurt the player either.

Mobs still don't seem to spawn at night. Some spiders were seen once. However, mob spawners can spawn them, and Just Blood Carts gives you a way to create Mob Spawners.