BibUpload: indicators in strong tags

* So far CFG_BIBUPLOAD_STRONG_TAGS config variable assumed strong
  tags had empty indicator. This was causing tags such as 999C0
  to never be matched. The new implementation assume strong tags
  specify also indicators. If they are not specified, then '%' is
  ussumed for ind1 and ind2.
  (closes #939)

Reported-by: Alessio Deiana <alessio.deiana@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>