`webacl.resource.getRights` does not return all rights if webId "system" is passed

I imagine that the `webacl.resource.getRights` action is intended to return the rights of a user. But it can also be useful to know all the rights of a resource, by passing a webId `system`. However, this is ignored and the user is considered anonymous: no rights are returned.

If we don't want to change the way it works, we should at least return an error when the `webacl.resource.getRights` action receives a `system` webId, because I think other developers after me would expect this to work.

Start by locating the webacl.resource.getRights action and trace how it handles the webId value "system". Clarify whether the action should return all resource rights or reject that value, then verify the chosen behavior with coverage for both the system webId and ordinary users.

Written by the indexing model from the issue text.