Update individual deadlines when the global one is updated

[Evgeny Kotelnikov]

A situation, occured in one of the courses:

1. A group got an extension for one of the labs.
2. The global deadline for the lab has been extended such that it is later than the individual deadline of the group.
3. The effective deadline of the group is before the global one.

What was expected by the teachers is that the latest deadline would be chosen.

Basically, it happens because we don't treat individual deadlines as "extensions" but rather as "overrides". I'm not sure if there is a use case when a group should receive a deadline contraction. If no, we'd rather indeed always choose the latest deadline.