Draft: Resolve "Changing user roles as a teacher"

Summary

When you setup the course as a teacher and you manually add people, there's no option for changing the roles after adding people (so like from TA to HTA), and you need to look up people's netIDs, and re-add them with the correct role which is a bit annoying.

Checklist

All items on this checklist should be checked.

• I have added a changelog entry.
• I have added a test that catches the original bug behaviour.
• I adhere to the style guide.

How to review

1. Log in as somebody allowed to add staff/students to an existing course edition.
2. Go to the course edition where you want to add the students.
3. Add a student as one of the roles.
4. Change the role of the student using the newly created button.
5. Observe the changes (including being moved between the student and staff page)

Closes #687