Exam Bank: Exam renaming: Backend

[some-molasses]

We should let admins rename the type of an exam, e.g., from "test" to "midterm".

There's frontend work defined here: #313.

On the backend, we'll need to set up a new endpoint that

1. renames the exam
2. regenerates the exams list (there's a function for this)

This endpoint should only be accessible to admins.

[ketjandr]

Hi! I'd like to work on this issue