increase configuration service robustness by providing default object

This change is intended to:
* clean up the code by removing the necessity to check wether the value is provided by the backend.
* increase robustness by having a default to fall back.
@ulferts use the fully-qualified `window.gon` rather than `gon` and you won't have to worry about catching a `ReferenceError` – you'll only have to deal with `undefined`..