mqtt: Add configuration setting for websocket port

A browser-based node spy will need to connect to the mqtt broker using websockets, so a second "Broker Websocket Port" or "Broker WSS Port" setting will be needed to support this.

Would it make sense to include in SCFG and store in main.ini? Or since this could be considered an optional feature, maybe it would make more sense as a modopts.ini setting instead?

EDIT: I guess there should actually be separate settings for WS and WSS ports, so sysops who haven't setup TLS can still make use of the node spy.

So I'd lean towards specifying the websocket ports via modopts.ini, which keeps the SCFG screen to the minimum needed for mqtt to work.

Then if/when someone loads the node spy, I'll have it output instructions on how to specify the websocket ports in modopts.ini.