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>