Calculate chance to drop using a slowly increasing function

This would address part of HabitRPG/habitrpg#1922 by giving a maximum drop chance of 75%.

The commit adds a function that increases as the chanceModifier increases till a maximum of 75%. It's based on a Monod equation https://en.wikipedia.org/wiki/Monod_equation

Feel free to ignore if this is not what you are looking for.