BibDocFile: get_icon() for smallest size icon
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) (closes #1743)

Signed-off-by: Ludmila Marian <e82cd4fb30962648dc4a4769a1bcc5e649b71d6b@gmail.com>
Reviewed-by: Samuele Kaplun <2e218f779e12714c314fad8ebf6b3041daa21337@cern.ch>

tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio