websocket port is hard coded

[Stuart Auchterlonie]

The websocket port is hard coded to 6554.

It should be changed to use <backend_port> +offset

The default set of ports is 6543, 6544, 6545, and 6554 for the websocket.

If you change the backend_port from 6543 to 7543 (for example) the allocated ports then become 7543, 7544, 7545, 6554

For development i run master on a non standard port, so once 0.28 is out, 0.29 will initially have a port conflict with 0.28

[Stuart Auchterlonie]

In caaaeef8166722888012f4ecaf3e9b0f09df512a

    The port numbers for the websocket (for the backend webserver),
    and the upnp ssl port were hardcoded.
    
    This changes the default port number to be
    - backendport + 5  (websocket port)
    - backendport + 10 (upnp ssl port)
    
    This leaves the default upnp ssl port unchanged, and moves
    the websocket port to one which does not conflict.
    
    This allows 2 instances of the backend to run simultaneously
    on the same system (ie. dev & prod), provided that the backend
    port for one of them is changed to a different one.

[Stuart Auchterlonie]

Fix included in fixes/0.28 as well