WebSession: dirty session support

• Adds a new property to the InvenioSession class called dirty, that can flag a session as dirty and hence worth being made persistent, in particular if it contains useful keys.

• Amended BibAuthorid usage of session.save() to use session.dirty = True, so that saving is performed only once per request.

Co-authored-by: Samuele Carli <samuele.carli@cern.ch>