Add support for the "viewer" role

This is both a stepping stone towards the "reviewer" role, as well as a full fledged roll itself. The "viewer" role would be owner managed, and allows read-only access to a dandiset. This is mainly applicable for embargoed dandisets, as otherwise the dandiset is publicly visible.

The addition of this role should probably involve a change in the UI/API, changing focus from "owner" to general "permissions".