Groups size extension

[Evgeny Kotelnikov]

Some courses are rather strict with the number of students in groups. Yet they want to make exceptions sometimes. The current approach, when you'd simply loosen the lower bound of the group size doesn't seem good enough.

A possible solution would be to add an extra relation between labs and groups with a boolean flag, that indicates that the group is excused to submit to tle labs regardless of its size.

[Evgeny Kotelnikov]

An extension should probably be discarded when a student joins or leaves the group.

[Evgeny Kotelnikov]

Implemented in develop, although an extension is not discarded when the group changes. I'm not sure how important it is. In any case, as we will remove the concept of groups eventually, the extension with be instead given to sets of students and this problem disappear.