MK3 Y Motor holder end stop

I incorporated this idea into an overall better and more complete solution, see [Taurus Y](https://www.thingiverse.com/thing:3269389).

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So TheZeroBeast had a brilliant idea to solve his Y axis crash recovery issue (see https://github.com/prusa3d/Original-Prusa-i3/issues/76). He submitted a pull request but no telling if/when that will make it in.

I didn't realize (should have) that the y carriage stops on the bearing on one side. There is quite a bit of range this bearing can be mo