ANW-1506 Display representative file version in PUI

[brianzelip]

This PR closes ANW-1506 by displaying a digital object's representative file version in the public interface.

Description

• Render image file uri in PUI digital object record
• Add representative_file_verson to objects controller
• Update locals passed to digital partial in objects show template
• Add representative_file_version template
• Refactor digital template around new rep fv template
• Add representative fv helper to resource controller
• Pass representative fv local in resource show view
• Add tests to digital_objects_controller_spec.rb

[quoideneuf]

We are breaking some existing functionality that used logic in the public controller and templates to work out whether to display the image or an icon based on various file version properties. But I am going to sort through that separately.