Raise3D N2/N2+ bed mod for leveling, stronger springs

I started with TobyCWood's excellent Raise3D N2 leveling mod, and made some changes.

Our Raise3D printer is probably newer, and has a fully insulated plate under the bed, so we could use metal washers instead of printed washers.

I couldn't source the same springs used in the original, so I used some much stiffer springs from McMaster Carr, and resized the stabilizer to match.

I also regenerated the wing nut as a slightly larger part, easier on the fingers to turn. I used 40mm flathead s