utils: etree to dict translation reorganization

* Adds xmlhelpers to utils to handle custom XML.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
utils: import json from arxiv api

* Adds API to import JSON data from http://export.arxiv.org with a
  given ArXiv ID.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
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>
travis: less verbose output

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
installation: requirement addition for six library

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
arxiv: docs entry addition

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: css theme overrides

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
registry: keygetter value fix

* `register` method of `DictModuleAutoDiscoverySubRegistry` class
  gives original value of parameter `value` to the keygetter method
  as well as the modified value from valuegetter.

* NOTE If your registry.py is using DictModuleAutoDiscoverySubRegistry
  and you use your own keygetter method then please update your
  keygetter method to use all three parameters.

Co-authored-by: Jan Åge Lavik <jan.age.lavik@cern.ch>
Co-authored-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Graham R. Armstrong <graham.richard.armstrong@cern.ch>
workflows: registry fix for keygetter method

* `keygetter` method of WorkflowsRegistry updated for keygetter fix.

Co-authored-by: Jan Åge Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Graham R. Armstrong <graham.richard.armstrong@cern.ch>
pidstore: new pid provider for record ids

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
uploader: first release insert mode
uploader: master format save to bibfmt

* PEP8 fixes
Uploader: enhance workflows with pre and post tasks
Docs: uploader initial docs added
uploader: documents module pluged

* Update celery version as is needed to solve `chord` and `starmap`
  problems.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
uploader: add model to documents
uploader: enhance field definitions