Stackable CD rack with front piece

Shout out to [Slice64](https://www.thingiverse.com/slice64/designs) for [the original awesome design](https://www.thingiverse.com/thing:5569597). This was a great first upload and inspired me to do mine!

Whilst printing this CD rack, I wanted a little extra structure in front so the empty slots are more rigid while they wait for me to expand my collection.