BibFormat: initial example of Twitter Card support

* Outputs sample Twitter Card markup (using fallback on OpenGraph
  markup).

* Uses DBCOLLID2TWITTERCARDTYPE knownledge base to map record type
  to card type.

* Introduced parameter 'one_icon_per_bibdoc' in bfe_meta_opengraph_image
  in order to avoid repetition of same icon preview in multiple sizes.

* Better output in bfe_meta_opengraph_image for Twitter Cards, with
  dedicated tag to comply with image size limit. Introduced
  'twitter_card_type' parameter to switch between 'gallery' and 'photo'
  card type (stricly either a group of 4 photos, or a single one).
  Reordered bfe_meta_opengraph_image and bfe_meta_opengraph_video in
  order to force Twitter not to fall back on corresponding OpenGraph
  tag (which would otherwise most probably get it reach the image
  size limit).

* Some additional CERN-specific changes.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
WebJournal: sample Twitter Card markup

* Added necessary tags to support Twitter Card.

* Use article featured image for Twitter Card image.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>