Quick release Y plates for Prusa simplified mendel (Q50682)

There aren't that many files around for cutting the small and large Y plates. Here's what I made. I went for the same magnets-plus-socket-cap system I used on the X carrriage, largely inspired by the Cupcake build platform. The largest plate was carved to save on weight and allow more leeway for the tensioners et al. (check out the "automatic belt tensioner", if there is ever some slack in the Y belt just slightly unscrew one the holders and it gets the axis back under tension. Will be checking how long the spring takes to cut through the Y belt loops of course ;) So far, all works reasonably well.