Electrical improvements to pi-hat board

I've had some ideas about electrical and layout improvements to the pi-hat board:

1. All the empty space on the board should be converted into a ground plane with additional ground traces running in-between the signal traces. One benefit is that less etchant gets consumed during the manufacture process because less copper has to be removed. The other benefit is improved EMI protection. I've heard one anecdotal report that if the authbox is brought close enough to a GAST air pump, the Raspberry Pi may spontaneously reboot. No idea if that was caused by the pi-hat board, but we do have long signal traces that can potentially act as an antenna.

2. Add room for a debouncing capacitor between each Bn_LOGIC line and GND. The board still works without them thanks to the `GPIO.add_event_detect(... bouncetime=150)` we have in software, but that's just a simple delay. If you hold the button down longer than 150ms and release it, it will register as a 2nd press due to bounce caused by opening button contacts. There's no need for separate resistors since we enable internal pull-ups in the Raspberry Pi for the Bn_LOGIC lines.