[#463] Add project_update_extra API resource

The project_update_extra resource adds some data from User and
Organisation to the normal project_update resource. This is needed for
the JSON-fetching plugin on akvo.org since it needs all data in one go
(or at least that makes things very much simpler).
[#463] Extend schema description of project_update_extra

Extend the schema with a description of the User and Organisation data inlined in the "user" field.
Merge pull request #469 from akvo/feature/463_akvo_web_updates_resource

Looks good to me