Allow more than 1 instructor to moderate annotations

## User story

As an instructor co-teaching a course, I would like my co-instructor(s) to have moderation privileges and the ability to grade students' annotations.

## Background

When our LMS app is installed, the first time an instructor-user in a course site creates and launches a Hypothesis-enabled reading, a group is created for the course and that instructor is set as the "creator," which comes with moderation and grading privileges. Many classes are co-taught, or have a TA enrolled as an instructor to help with grading, etc. Ideally, all instructor-users in a course site will be able to grade and moderate annotations.