deposit: FFT from arXiv

* Gets the FFT from arXiv if the arXiv id is given.

Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>