Lab Rax 10" Server Rack Right Angle Edge for 1/2U

Thanks [mklements](https://makerworld.com/en/@mklements) for the great 10" server rack design. This is a remix of the horizontal edge of the original design for some extra functionalites:

1. Right angle is more stable preventing the cage from tipping forward, especially, when most of the parts are mounted on the front half of the cage.
2. I made a notch so that we can claim another "half U" space – if you do this with the top part, you can get one extra unit height.