Stackable DVD Blu-Ray Wall Rack

Update 2/13/19: New and improved version available: https://www.thingiverse.com/thing:3425351

Created from scratch based on a design by tmorris9, but such that multiple racks can be lined up seamlessly without a gap between the discs.

Other changes:

* Reduced structure thickness (for quicker printing with less filament, still more than sufficient for it's purpose)
* Reduced the size of the slot for the disc to keep the discs securely in place (felt too loose in the original design, tho