Improved Promega CoreXY Belt Tensioner

This is a remix of supinemonkey's excellent Promega belt tensioner mod that improves upon his design. The improvements are:

1. The horizontal mounting hole on the knob bracket has been reduced to an M4 thread due to the holes in the Promega frame being too small for an M5 screw.

2. The NEMA motor spacer has been flattened to accommodate a 10mm threaded standoff in order to reduce or eliminate the distance the motor pinion has to be moved up the shaft. The new spacer places the motor within