Allow serving web interface and REST API on the same port

[kroepke]

Now that #2440 is merged, we can allow serving the web interface and REST API from the same port.

The suggestion is to allow setting the web_listen_uri to the same host/port combination as the rest_listen_uri.
If that's the case the web_listen_uri needs to verify that a path different than / is set.
The resources of the web interface are then added to the same API service that serves the REST endpoints.

Otherwise a second API service is started, which functions exactly as it does right now (i.e. separate port is bound, the two API services are different).

The current behavior should be the default to avoid surprises for current 2.0 users.