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 puzzle.

You can now also change the depth and size of the embedded initials, and opt to leave out the logo on the inside.

If Customizer is broken on the Thingiverse website, [here are instructions](https://www.thingiverse.com/thing:2812634) to run it on your own computer.

It is possible you'll still get a non-manifold model if you try outlandish settings in Customizer (especially if you use a low number of steps), but generally it should be OK.