Break permission inheritance by adding the none keyword

Eg.
- `{read: [system.EveryOne]}` on bucket
- `{none: [system.EveryOne]}` on specific collection in this bucket
Users can read every collections in bucket but one specific.

Inspiration: Syncano permission types http://docs.syncano.io/docs/permissions#section-permission-types-for-data-objects-

No implementation files or tests are named. Start with the issue's bucket and collection examples and the linked Syncano permission-types reference, then locate Kinto's permission inheritance logic. Done means a none permission on one collection prevents access inherited from the bucket while other collections retain the bucket permission.

Written by the indexing model from the issue text.