Ticket/master/pdb 954 fallback acceptance

The url-prefix setting is no longer necessary is it when we move to server_urls? Also - there is a link to this from the url-prefix setting over in the configuration page that we might want to change to reflect this.

I tested the prefix still works, and that it doesn't interfere with the new behaviour also and it seems fine. However, no deprecation message there either (same for server/port atm).