Make the base-2013 index template respect the configured homepage

`config.yml` provides a `homepage` option to set the record that is passed to the index template. However, the `base-2013` theme ignores this option and instead always displays `page/1`. This commit removes the hardcoded resource from the theme and instead makes it use the global default.

If I'm missing something and there's a reason that the theme behaves this way, please tell me.