Sticky visibility setting

- AnnotationController.save() saves the used Privacy into localStorage
- AnnotationController.edit() sets (if given) the stored privacy for root annotations
- Minimal wrapper around localStorage access as a service called: localStorageHelpers

Fix #1299 

> Thanks for extracting these. I think it would make sense to make them
> properties of |localStorageHelpers| as this is where the code that
> uses them is.
>
Done