NRF52840 DK prototyping rig with button support

This is basically the same as the source prototyping rig, which is very nice. I noticed when using it, though, that the bottom right button (Button 4) caused the board to move around when pressed because there's no screw hole to its right, so I added a pad to support that side of the board when pressing the button and rearranged the base to attach to that support. I had to recreate it in FreeCAD to do it, so I also provided a simple template file with a solid base pad and supports in the right place.