No breakout for GND on bottom header row

[jeffellenbogen]

In wiring up a test circuit for allow for a fan to be controlled via the microbit_led_translator board I was able to solder the external transistor circuit directly to P0 and 5V along the bottom row of headers. However, GND is not available. Instead I used one of the GRN headers that was intended for LED strips connected to P0, P1, or P2. It would be great to have GND available along the bottom row as well.