BibDocFile: get_icon() for smallest size icon

 * Updates get_icon() function to return the BibDocFile
   associated with the subformat CFG_BIBDOCFILE_ICON_SUBFORMAT_RE,
   or, if this does not exist, the smallest size icon available.
   (closes #1350)

Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Signed-off-by: Ludmila Marian <ludmila.marian@gmail.com>