deposit: improvements of literature deposit form

* Changes Subject field to SelectField with data
  choices populated from knowledge base.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
deposit: add subjects mapping

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
deposit: add configuration for subjects KB

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>