Feature capped exam authorizations

[flbulgarelli]

:dart: Goal

To allow exams to have a cap of students per turn.

:memo: Details

This feature introduces:

• available_exams which contains most of the magic here
• authorize_requests! and update_request_by_id! which may look new, but is actually old code extracted from laboratory

:back: Backwards compatibility

Backward compatible, as long as you run migrations :stuck_out_tongue:

:soon: Future work

It would be nice to a more detailed exception than just Gone which actually states that the exam is full.

[flbulgarelli]

@Gustrucco @lauramangifesta