Create an Opencast group for Sakai instructors

To facilitate finer-grained permissions, it's helpful in some contexts for Opencast to know which Sakai users are instructors, meaning that the user has an Instructor-equivalent Sakai role (Site owner, maintain, etc.) in one or more Sakai sites.

So we will add a new group, ROLE_GROUP_SAKAI_INSTRUCTOR and the Sakai user provider will add this group role to users who meet the criteria.