Assign the same graders to connected labs

[Evgeny Kotelnikov]

Sometimes labs, albeit being separate exercises, form continuous assignments, where the outcome of the previous lab is used in the subsequent one. It would be nice in such cases to assign the same graders to the same submitters.

In some courses, all the labs form a sequence, in others — only some of them.

Questions to answer in the implementation: - What to do if submitters of the second lab didn't submit to the first one? (do not allow that?) - What to do if submitters of the first lab don't submit to the second one? It screws up the distribution of grading workload. (do not care about it?)

[Evgeny Kotelnikov]

Available in master, with an interface in develop.