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.