allow use of (https) reverse proxies by default

[patagonaa]

Use the snapweb URL for connecting to the websockets by default.
This allows running snapweb behind a reverse proxy but might break existing installations if snapweb is not running in the snapserver's web server.

fixes #21

[badaix]

In which scenario this change would break an existing installation and how could this be fixed?

[patagonaa]

If you're running snapweb under a different web server than snapcast (like, say, snapweb on port 80 and snapserver websocket on 1780) this PR would break the installation (because snapweb would try to connect the websocket to port 80 instead of hard-coded 1780 like it used to).

I'm not sure if this is something people do/did, and the solution for them would be to either move snapweb to the snapservers' web server or to edit their config.js

[badaix]

Understood, thanks 👍