deposit: define 980__a value based on form information

* Depending on which information in the form is filled and what
  document type selected we should add different 980__a values.

* Adds 980__a:conferencePaper if confefence information is given.

* Add 980__a:Published if journal info is given but no conference info.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>