Support for "copy ids"

[nthiery]

Add support for "copy ids": a short id (~3 characters, typically the
student's seat number, or ...), reasonably unique within the cohort, that
the student has to write down on all sheets so that, if all sheets gets messed
up, it is still possible to semi-automatically regroup sheets together.

This avoids repeating all of the student info on all sheets (which uses space
and student time).

About "reasonably unique": there always will be the case of students misspelling
their id, ... So some sheets might need manual treatment like checking the
hand writing to finalize the regrouping.