Revamped Pi Rack Chassis

All credit to the original creator of this. I've made a number of changes, which I'll go over briefly.

The carrier for the sleds had a cable management arm that required supports and I didn't find it useful, so I removed that. The carrier has a large number of artifacts. I fixed some, and it prints ok, but there's a number of small gaps and oddities in the final print, none of which affect functionality.

There are two versions of the Pi sled. The one with the square cutout in the center is for just a plain Pi, with the standoffs moved to match current rev Pis. I also increased the width by about a millimeter so it would stay in the carrier's tracks. The Rev B sled (with the round holes) is for use with Pis that have the PoE hat, and allows you to insert a screw from the back to use the standoffs that come with the PoE hat. Note you'll have to remove the fan from the PoE hat. I recommend heatsinks on the processors (see picture)

I've also redone the rack ears. They're