Simplify setting of the URLs for hosts, allowing PUBLIC_URL environment variable so that it can be set from awsbox

This changes us to not need to set `ROOTURL` and `WSURL` in the configuration files, but keeps the capability, e.g. for local configuration changes.

`WSURL` is now specified based on `ROOTURL` if appropriate.

`PUBLIC_URL` can now be set in the environment, and this will set `ROOTURL` (and hence `WSURL`).

Its conceivable in the future we might want to be able to specify the `WSURL` in the environment but that's unclear yet, and we may have a easier way to set more environment variables by then.