instance staticpages

This allows to configure additional staticpage api addresses for specific instances.

If an instance has a custom staticpage api address we try to stay in that instance when navigating to staticpages. Therefore the `entity_url` and `external_nav` are adapted.

Note that these static pages replace the global ones in the UI. This is not about *additional* static pages on instance level, but about *replacing* the global ones in some instances.