course: make it possible for an instructor to change the cocalc account associated to a student

[williamstein]

Right now in the course management interface every student has an associated cocalc account (if the student has created an account already). There is no feature to just change that account through the UI. I've done such a change a few times using vim by just editing the .course file directly. Here's a situation where this comes up:

1. Start the class and everything is fine for the student, who already has an account A with email foo@example.edu.
2. For mysterious reasons, the student panics. They student changes the email address on their existing account A from foo@example.edu to bar@example.edu.
3. The student then make a brand new account B which has nothing to do with the course at this point, but does use the email address foo@example.edu.

At this point there are two options:

• student goes back to using account A.
• FEATURE REQUEST: instructor changes the course file to instead define the student's account as account B. This could be done via the same sort of "select a user" search that is in the new messages code:

[williamstein]

NOTE: This class of problems would disappear whenever a university integrates cocalc with their SSO system. The drawback is that (1) this involves a lot of bureaucracy, and (2) can introduce other problems, especially at places that have a lot of accounts already.