Bug-A-Salt Sling Holder

This is a full fledged bracket sling for the [Bug-A-Salt 2.0](https://rb.gy/9pi6b3) gun. I am using it to hang it off the side of my fridge, as shown in the last picture. The other two loaded pictures show the two placements:
* centered, which is balanced, but doesn't hold the gun as well
* locked, which is off-center, but holds the gun tight. The bracket does not hang balanced.

This design is fully parameterized, but these dimensions seem to work really well, and fits on an Ender 3 (220mmx220m) when rotated 45 degrees on the bed. If your bed is smaller, you can adjust the br_space variable, but you'll have less stability holding the gun.

The design is in two pieces, the holder and the arm. The arm is designed to allow a strong magnet (mine is pulled from an old hard drive) to hold the bracket tightly to the top of the fridge, as shown. Tape on a clean surface would also work well. The two pieces mate with a joint, so if you have some different mounting need, feel free to desig