Add blink_leds request
Add blink_leds request

Leds can be blinked by issuing a POST request to the bt.tn socket
server with the following payload.

```
auth = MY_PASSWORD
blink_leds = ffffff;400;0;400;123456;400;0;400;321299;400;0;400
```

First field tells the shift register which leds to light up, second is
the delay before issuing the next shift. This pattern repeats, e.g.
`LEDS;DELAY;LEDS;DELAY;LEDS;DELAY`.

mafredri/openbttn,mafredri/openbttn,mafredri/openbttn,mafredri/openbttn,mafredri/openbttn