Implement Design and Drag'n'Drop for Feature Graphic

App edit mode now allows to add a feature graphic. However, this is just basic and needs to be implemented the same way that screenshots work with the exception that there can only exist one feature graphic at a time.

We probably should show the feature graphic also in the app details screen somehow somewhere.

The implementation should come with view tests that ensure that the feature graphic is saved properly and that the old one is deleted when a new one is saved.