delta (kossel mini) vertex for 2020 extrusion with motor shaft support and tensioner

When at some point my delta vertices got distorted thanks to the heated bed, time and belt tension I thought it's time to design new ones and address the issues I observe. Namely, 1. the thing bending so that motor shafts look a bit upwards, due to belt tension (added a bearing to support the shaft on the other end), 2. somewhat ugly way to to tension belts that distorts geometry a bit, 3. aesthetics of the circular corners (a matter of taste and the `.scad` files provided still have an option t