ffwizard-berlin: add support for private encrypted APs

SSID and key fields should be not visible until the user enables the checkbox. This should be manipulated via javascript for example in the following way:

```javascript
var hideElements = function() {
  var displayStyle = this.checked ? "block" : "none";
  document.getElementById("cbi-ffvpn-1-private_ap_ssid").style.display = displayStyle;
  document.getElementById("cbi-ffvpn-1-private_ap_key").style.display = displayStyle;
}

window.addEventListener('load', hideElements, false);
document.getElementById("cbid.ffvpn.1.private_ap").addEventListener('click', hideElements, false);
```

For now I don't know how to put raw javascript into the template...

See as well https://github.com/freifunk-berlin/firmware/issues/40 for more information.