Added better support for CanCan#accessible_by

* Added an `action` optional argument to AuthorizationAdapter#scope_collection. The controller passes in the action that is currently being performed so that authorization adapters can correctly scope collections based on different controller actions.
* Implemented it on the `CanCanAdapter` so that we offer full support of the accessible_by functionality.
* Removed the collection scoping that was happening when finding the resource. It should only happen when retrieving the collection.