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>
Analysis: Deposit form creation [unfinished]
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