Added User::singleUserNickname() as (temporary?) fallback for single-user lookup...

Added User::singleUserNickname() as (temporary?) fallback for single-user lookup as a workaround for site setup of 1user sites. We found that an external tool attempting to spin up StatusNet and then register the user would fail because StatusNet's router setup dies on being unable to find its single-user account, since the nickname is needed in setting up routing entries. This tweak will let it survive, using the configured setting as a fallback if it can't actually find the user account.