Server: Let teachers use user() query on their students

[TiE23]

Teachers should have PrivateUser access to their students. Write smart solution to this that keeps things efficient for cases where Teachers are looking up students.

Modify the query user() and users() to allow a teacher to do this.

I would highly recommend making the code that discovered if a teacher is a student's teacher into a utility function because there are multiple places where this would be useful (for example, the mutation assignCourseNewSubSubjects()).

I would recommend after completing this ISSUE to write a ticket that covers additional mutations/queries where this feature would be useful... or of course, just feature-creep this ISSUE, lol.

Do before #91.