apps.core.models.assignment_group: Added group_by_examineruser().
apps.core.models.assignment_group: Simplify group_by_studentuser().
apps.core.models.assignment_group: Fixed variable naming in group_by_studentuser().
apps.core.models.examiner: relatedexaminers_by_tag and

relatedstudents_by_tag support filtering by user objects.
apps.core.models.examiner: Working setup_examiners_by_tags().