Modular project box shelves

This is set of connectable shelves currently sized to fit [these small project boxes](https://www.amazon.com/gp/product/B004T6I6RA) but it would be easy to change the dimensions at the top of the OpenSCAD file to store other things, or change the angle or number of shelves.

You'll need to print one of the following:
* left_end_plate.stl
* right_end_plate.stl

and one per shelf column of the following:
* left_side.stl
* right_side.stl
* back_bottom.stl