Show mixing HTTP/HTTPS warning

[shrimpza]

If you're hosting/accessing Aurial via HTTPS, typical browser security will prevent you from accessing a Subsonic server over plain HTTP.

Likely the only solution to this is to pop up a warning, informing users of this, so they can change one side or the other to HTTP or HTTPS.

This is particularly evident when trying to access the demo page at https://shrimpza.github.io/aurial/.