Add support for group-based assignments

[chrfrantz]

This is a bit bigger request and we can discuss what is feasible: After reflecting with other lecturers: We should have a different kind of assignment type: a group assignment. Students can sign up to a group (or create one) and submit together (i.e., any member of the group can submit and edit the submissions). The group may then be involved in reviewing other groups assignments (Example: 3 reviews per group assignment, with flexible choice of which group member is doing the assignment - e.g., one member could to all, or could be distributed - this would be self-managed among students). All members of the group share the resulting marks.
Fundamentally, the only abstraction is to have people operate over group relations (i.e., check for group identifier when submitting, reviewing and providing results; the other dynamics of the systems would be unaffected.