Display inconsistency when extending deadline with existing new submission

[Gregoire Detrez]

In the situation where a submission has been rejected and the grader tries to extend the deadline for a group but there is already a new submission, the deadline is correctly changed and correctly displayed to the student but the grader sees the old deadline.

Solution: move the deadline extensions to a separate relationship?

[Evgeny Kotelnikov]

Resolved by one of the latest commits.