[SHELVED like RFC #12] Allow explicit (additional) permissions

[fenekku]

Part of this user story:

As a record owner or curator, I want to be able to give permissions to specific individuals (or groups eventually) so that those individuals can perform certain actions.

Implement Generator that can interpret the additional permissions field : because access_rights and permissions are intertwined we merge their generators together. Striked out because the additional permissions are additive for now. If we consider subtractive (bans) then we will have to talk about the implications with respect to access_rights.

Implementation of RFC: inveniosoftware/rfcs#12