deposit: duplicate validator with prod inspirehep

* Queries inspirehep.net to check if a submited publication is
  already in the production db.

* Introduces PRODUCTION_MODE variable which enables cooperation with
  inspirehep.net server.

* NOTE Set PRODUCTION_MODE=True in your config to use the validator.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>