Students and Examiners tied to Periods

[espenak]

To make it easier to distribute students among examiners on an assignment, we should have a list of students and examiners associated with each Period.

In most cases the list of users will only be used when creating the first assignment, and then the results of the first assignment will be used to distribute students between examiners when creating the next assignment.

But there are special cases, where the results of one assignment do not affect the next assignment, and where not all students are taking all assignment.

To cover these cases without making the system so flexible that it is unusable for "normal" use, I propose that we use the proposed metadata system, and leave it up to the system administrators how the choose to use it. I believe the typical usecase to be that students and examiners are dumped into a metadata string list, and the webinterface enables the use of such lists when distributing deliveries.