Renamed contentType to resourceSubType

- Renamed contentType property to resourceSubType to be consistent with the output of search. Not sure it's necessarily the best name, but it's now easy to change across the board.
- Renamed sakaidocs to documents in anticipation of collaborative documents. Will submit a PR to the model loader as well.
- Made documents private by default

NOTE: This should not be merged until the security test is over, as it introduces a data model change