documents: set_content and resful API

* Adds new model and field definitions.

* NOTE: this is still work-in-progress.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
jsonalchemy: versionable extension

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>