Refactor document overview

Refactor document overview to remove logic from templates.

The set of fields that are rendered in the template is now defined by a method `get_metadata_config` on the `Overview` view that can be overriden in subclasses.

This is *mostly* a refactoring, but there is one functional change: In the new implementation, all fields returned by `get_metadata_config` are now displayed, whether they have a value or not. This simplifies the implementation and makes the UI more consistent - the set of displayed fields is always the same.