Refix #950: thumbnails not showing overview upon click in widgets UI

Fix for #991 lead to having all thumbnails matching the 'unknown'
case - thus not differentiating large_thumbnail from thumbnail.
Add getOverview json converter and display the overview upon clicking
the thumbnail (if it exists)
Fix the iso19139 schema to properly index all thumbs/overviews.

This should also apply fine on 2.8.x branch.