Fine-grained permission control through id

[Ljinod]

(following @Gara64's issue #204)

In order to share files between cozy we would need to be able to define fine-grained permissions based on the id of a document instead of the, more generic, docType.

The reason behind this is that with the actual configuration if a user wants to share, for example, an event from his calendar the sharing process needs access to the "event" docType, thus to all events. We think this behavior is too permissive (for our purpose) and might lead to unwanted access to other data.