Add warning if default Wi-Fi password is used

Created by: JoKeyser

While de-bloating the first welcome screen (#830 (closed)), the warning about default passwords got scrubbed. Clearly it's important, and should always be present, regardless of first boot; the question is where. Since that's a more general question (see #849 and card-based UI) that isn't resolved yet, a temporary solution might be to add a warning message in the network settings, if the default WiFi password is unchanged.