knowledge: REST API addition

* Implements a first prototype of REST API to deprecate
  old kb export. (addresses #2570)

* Adds a testsuite for REST API

* Deprecates some duplicate methods in the models.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
authors: base record

* Defines fields and models for author base record.

* Adds new function for jsonalchemy models definitions.

* Includes tests for author base record.

Signed-off-by: Mateusz Susik <mateusz.susik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
oauth2server: fix support of SQLAlchemy-Utils

* Addreses issue with new API of EncryptedType in
  SQLAlchemy-Utils>=0.28.2.  (closes #2629)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
installation: python-twitter>=2.0

* Updates python-twitter in requirements.txt to the latest version
  (2.0) that supports Twitter 1.1 API.  (PR #2015)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
indexer: add admin interface

* Adds a admin interface for indexer developed with
  Flask-Admin.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>