Prusa i3 reinforced Y motor mount

This is a remix of Xoan's thing #85809 which is itself a remix of Josef Prusa's original Y motor mount for the Prusa i3 (with 8mm threaded rods). It provides three attachment points to the NEMA 17 stepper vs. only two in the original Prusa i3 part.

It is compatible with any Prusa i3 using 8mm threaded rods (for example, Twelvepro's improved Prusa i3).

I have smoothed out and reinforced Xoan's part (eliminating what I thought could be potential stress points) and cleaned up the source code