Slim 18650 Fatshark Case with Cell Checker

I remixed dmcke5's excellent [18650 Fatshark case with cell checker](https://www.thingiverse.com/thing:1908636) with the following goals:

- Combine bottom and body into a single piece
- Secure top with M2 screws
- Reduce the need for supports by flattening top and bottom (I prefer to keep the pack in my pocket with an extension cable)

I used redrobrc's [taller button](https://www.thingiverse.com/thing:1959769) in TPU to reduce rattling and make the button easier to find.

#### Non-Prin