ROOT_URL setting use the default as shown in conf/app.ini (#1823)

ROOT_URL setting use the default as shown in conf/app.ini (#1823)

The well commented conf/app.ini file that comes with the code shows the
ROOT_URL (i.e. setting.AppURL) as:

    ROOT_URL = %(PROTOCOL)s://%(DOMAIN)s:%(HTTP_PORT)s/

However the installed custom/conf/app.ini file does not include this setting as
shown, and the default in the setting module was hard coded to
http://localhost:3000/ instead of what is shown above.

With this change the ROOT_URL will default to what is shown above if it is not
set in the custom/conf/app.ini.

Of course it is still possible to override the default by adding the ROOT_URL
setting to your custom/conf/app.ini file as usual.

Signed-off-by: Mike Fellows <mike.fellows@shaw.ca>