A permission system for RDF

We may want to extend the `django-contrib-auth` permission system to RDF data, for example

- encoding group- and user-level permissions in RDF (when a particular permission is required to perform a particular operation);
- encoding object-level permissions in RDF (when authorization depends on who "owns" a particular IRI).

Since RDF has soft rules, the only viable default is "no permission at all"; we can then store in RDF that somebody has a particular permission by way of an exception.

Depends on #80, #85. May also publish authorization-related information, which depends on #81.

This is a research topic; if we decide to actually do something like this, we should create more specific issues.