Single beam coupling supported on "Plastic" self aligning thrust bearing for Prusa

This is a remix of thing 11891

By adding a 20mm ID collar to thing 11891, it is possible to use "plastic" M8 self aligning thrust bearings from Igus (RS part number 712-5011), these bearing offer very low friction and unlike the ball race type thrust bearings originally used with thing 11891 allow for miss-alignment compensation. I also chose to use aluminium single beam couplings, which need to have the weight of the x-carriage supported so that the couplings do not stretch.