Possible data invariant violation when student changes groups

[Evgeny Kotelnikov]

Let's say students A and B, working correspondingly in Groups 1 and 2, submit to the same lab. A got accepted, B got rejected. Later on A joins group 2 to work together on other labs. B still has to resubmit, but she cannot do it, because the resubmission would make both A and B submitters of the Lab 1 and it violates the invariant, that says that students cannot have submissions to the same lab as members of different groups.

This is a real situation with a very nasty workaround and something should be done about it.

[Evgeny Kotelnikov]

Fix #55

→ <<cset b7ba3eac6841>>