Customizable Twist Box (improved model)

This is a small improvement upon RevK's customizable [Twist box](https://www.thingiverse.com/thing:2272576). The model is now manifold, which means slicers (like Slic3r) won't have problems with it. I also added some extra options to the [Customizer](https://www.thingiverse.com/apps/customizer/run?thing_id=2545312), most importantly the option to incorporate small nubs that cause the two halves to snap together when closed. This makes it less obvious how to open the box when considering it a puz