Display the image in case of Reference fields on admin UI

[tusmester]

As an editor I want to see the actual related/referenced image when editing or browsing a content.

This issue is only about the field control for the Reference field type.

• single and multiref reference fields
• Browse mode:
  • display the image(s) instead of the current icon (may be bigger)
  • Maximize the image box size to a square
• New and Edit modes
  • if new referenced images are added, display them the same way
• display images only for image types, based on content type (if available) or extension
• if a thumbnail exists --> display that, otherwise fallback to the preview or the original image (there is an api for getting previews and thumbnails)

Out of scope

• CTD switch on the Reference field LATER
• upload new images LATER