Remixed case for Libre "Le Potato" using M3 threaded inserts

I liked @Justin's remix, but found it a pain to secure the M3 nuts from the top of the board since there's not a lot of room to hold them in place.  This remix modifies the holes in the bottom of the case to accept threaded inserts so that you can screw the board in from the top down rather than from the bottom up. Use M3x6mm screws to secure the board to the base. The lid is unchanged, I just included it here for completeness.