deposit: conference and proceedings info both in 500 field

* The conference information goes in one 500 field and the proceedings
  information in another 500 field.

* Temporary solution until we do a fancy conference lookup widget: we
  rename 'Conference name' to 'Conference information' and inside a
  placeholder: 'Conference name, acronym, place, date'.

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