Add strobe LED button on web dashboard

[AustinShalit]

This would allow users to identify which Romi they are connected with.

[ThadHouse]

The green OK LED can be controlled from software. It's available as /sys/class/leds/led0/

[zhiquanyeo]

https://www.jeffgeerling.com/blogs/jeff-geerling/controlling-pwr-act-leds-raspberry-pi

The instructions for the Pi2 might work for the Pi3 as well