Add protected extras

[maudetes]

First part of https://github.com/etalab/data.gouv.fr/issues/838.

Questions and TODOs

• [ ] resource protected extras route needs dataset id -> but not always available
• [ ] how to register additionnal extras (used in plugins for example)
• [ ] add protected_extras to serialize in dataset/events.py and search
• [ ] should we add protected_extras on discussions, users and orga ?
• [ ] move all to Harvest (remote_url, dct:identifier, etc..) ?
• [ ] what about linkchecker extras ?
• [ ] what about csv-export:model ?

[abulte]

Extra thought: since we're using HATEOAS, maybe we don't even need an embedded document and can use a full-fledged Document? It it helps making the code more readable, obviously.

[maudetes]

Has been treated in https://github.com/opendatateam/udata/pull/2762