CL-260 Ultimaker Clone Corner

This is a remake of [ForenSeil's CL-260 Corner](https://www.thingiverse.com/thing:2100073) which samella used in his [El Cheapo UM clone](https://www.thingiverse.com/thing:2336253) that I'm also building.

The STL of ForenSeil's part doesn't seem to be a correct mesh. Perhaps the original injection molding STLs were mashed together and they don't join well? Anyway, I could not modify it how I wanted so I remade the part in OpenSCAD.

I also remade the motor mount without the thicker piece wh