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>