Record the reason for student exceptions

[Evgeny Kotelnikov]

A request from Erland Homstöm

In the page "Group size exceptions" it would also be nice to have a comments field where I can write down the reason for working alone. The student doing his bachelor work in Amsterdam should be working alone for instance but the one who just did not find a partner should not. That is a big help when trying to put people in groups.

Similarly, we could add a reason to deadline extensions.