Stronger Z End Stop Mount for Folger i3 2020 8mm bar

The end stop mount that comes with the Folger i3 2020 has *two deficits*:
* It wiggles just a bit: this is not ideal when wanting a rock-solid z-axis end stop for reliably stopping in exactly the same place.
* The limit switch mounting holes should be closer to the edge of the plastic to be absolutely sure the switch triggers before the machine's carriage hits the z-axis end stop mount. Credit goes to **animoose** for noticing this problem and iterating an improvement at http://www.thingivers