Server: Look into updating schema that protects students in classrooms

[TiE23]

There is some interest I have in figuring out how to make the Classrooms system work better.

First, Classroom queries only support the type User, not PrivateUser, so a Teacher cannot get sensitive details on their students in a single general query like the one for query classroom(), I'd have to chain the results together and get them later (see #90 for my plan to make query users() accessible for teachers on their students).

Anyway, there must be a way to do at least one of the following two things:

• Let teachers get PrivateUser through a query for a Classroom.
• Prevent students from looking at fellow students (not just no PrivateUser, no User at all!) through a query for Classroom or through a user() or me() query.

Do after #90.