re #VIV-356: fixed teaser image URL computation, made it aware of videos
re #VIV-356: fixed teaser image URL computation, made it aware of videos

ImageGroups were used wrongly and thumbnail images of videos aren't content
objects anyway so they need some special treatment.

There are no tests for the fixed behaviour since the whole teaser story is
expected to get implemented as a concept in the near future.

ZeitOnline/zeit.newsletter,ZeitOnline/zeit.newsletter