re ZON-3171: Introduce constant for key of infographic display type.
re ZON-3171: Introduce constant for key of infographic display type.

- testing whether a image group is an infographic or not will always require
  comparison to this key

- to avoid duplication of the key to many files, introduce a constant

ZeitOnline/zeit.content.image,ZeitOnline/zeit.content.image