Walker skid foot

This is a remix of @captmurphy's design (https://www.thingiverse.com/thing:4729843).

Skid/foot changes
- Recessed the screw hole to the full depth of the skid thickness, so that you can't wear the foot down to the point where the screw head is exposed.

Expansion plug changes
- Up-sized by 10%. For my walker I found that the original plug fit inside the foot barrel without expanding it enough to grab the inside of the walker leg tube. My walker's tube is about 26mm ID.
- Captured the nut by making a side slot and capping the top-side. This change allows the plug to be tapped out of the foot by loosening the screw a bit and tapping on it.

Printing
- You'll probably want lots of solid thickness on the bottom of the foot to allow for wear without getting into infill. I printed the these with 30 bottom layers at 0.2mm layer thickness. I printed in PLA with supports on the front edge, rear edge, and in the screw hole. I tried ASA but the print failed due to leading edge o