Store WAC permissions in their own named graph

[srosset81]

In the current implementation, WAC permissions are all saved in a custom SemApps-specific named graph. While it would be technically possible to keep this with NextGraph, the standard-compliant way to do this would be to store WAC permissions as regular resources, in their own document (and thus their own named graph)

This will require quite a lot of refactoring. It should probably be done closely with #1189

• Automatically create a WAC permission every time a new LDP resource or container is created.
  • Put it on the same container as the resource ?
• Automatically link it to the LDP resource with the acl:accessControl predicate
  • This is the one used by StartinBlox
  • This predicate is described like this in the ACL ontology: "The Access Control file for this information resource. This may of course be a virtual resource implemented by the access control system."
• When a LDP resource or container is deleted, also delete the WAC permissions
  • If ActivityPub Tombstones are activated, the WAC permissions should be erased and we should only keep a public read permission.
• Migration tool
  • Go through all LDP resources and containers, create a Document with a WAC permissions. Look at the existing permissions of the resource or container. Add a acl:accessControl link from the resource.