arxiv: initial Flask extension commit

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
records: API for database querying with DOI

* Adds API to query the database with a DOI.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>