Add HASS status sensor + improve HASS discovery

[emontnemery]

Announce a status sensor for each device as in the below screenshot.

Also improve Hass discovery:

• Only announce the device in the status sensor, to avoid overflowing the MQTT message buffer
  With this change, full topic and friendly name (32 characters) is supported without any issue for all device types (worst case is RGBW light).
• In case there is again issue with overflow in the future, add helper try_snprintf_P which:
  • Detects and prints an error message in the log in case the buffer is too small
  • Makes sure invalid JSON is never generated