fix(console): disable the unauthenticated serial console by default

socketiod does not validate the console token, keeps a single global PTY
and broadcasts its output to every connected client, so anyone who can
reach port 6081 can read and write a serial console that someone opened.
It is also broken for legitimate users, since console/views.py now writes
signed tokens that socketiod cannot parse.

Until it is rewritten, turn the serial console off by default:

- Add SERIAL_CONSOLE_ENABLED = False to the settings template; installs
  whose settings.py lacks it are treated as disabled.
- The console view shows an error instead of the xterm page for pty
  consoles while the setting is off.
- socketiod refuses to start unless SERIAL_CONSOLE_ENABLED is True.
- Supervisor: autostart=false for socketiod; drop -d (debug) from the
  supervisor and systemd definitions; fix the systemd unit description;
  conf/systemd/install.sh no longer suggests enabling socketiod.
- README: stop opening port 6081 and document how to enable the serial
  console on a trusted network.

console/views.py

@@ -133,6 +133,16 @@ def console(request):
     if console_type == "vnc":
         console_page = "console-vnc-" + view_type + ".html"
         response = render(request, console_page, locals())
+    elif console_type == "pty" and not getattr(
+        settings, "SERIAL_CONSOLE_ENABLED", False
+    ):
+        # socketiod does not authenticate connections yet (ROADMAP S-01), so
+        # the serial console stays off unless an admin explicitly enables it.
+        console_error = _(
+            "Serial console is disabled on this server. "
+            "Please switch VM graphics to VNC or ask an administrator."
+        )
+        response = render(request, "console-vnc-lite.html", locals())
     elif console_type == "pty":
         socketio_public_host = getattr(settings, "SOCKETIO_PUBLIC_HOST", None)
         socketio_public_port = getattr(settings, "SOCKETIO_PUBLIC_PORT", 6081)