Expose uri and private_url for DatabaseUsers

In the default DB credential secret, there are fields for the uri and private_uri of the database, which is helpful to expose as env variables in containers. But these two fields are missing for additional users created via a DatabaseUser. This makes it rather more annoying to get the uri to connect to into an environment variable for a container without also exposing the admin password. (Though you can get the pieces as seperate variables but that may require code changes to our apps if they are expecting a uri.)