Gridfinity Glueless 3u bin

Remix of the Gridfinity 3u storage by Zach Freedman (https://thangs.com/designer/ZackFreedman) to eliminate the need for gluing magnets in place. Magnets are now inserted into press fit slots. In addition, magnets have been added under the label deck to allow stacked bins to hold.

All of my Gridfinity designs are captured in [my Github Gridfinity](https://github.com/doug-harriman/gridfinity) project.