BibFormat: special handling of INSPIRE-PUBLIC type

• Handles new INSPIRE-PUBLIC document type that denotes public INSPIRE documents (used to be empty doctype) in the Files tab visibility decider and in the full-text snippet courtesy message generator.
  
  (Note that this should be make more configurable so that the recipe how to generate courtesy messages out of document status, document type, or some other document property values are easily customisable.)