AssessmentGroup should not allow OnlyIfHigherThan reference to grow deeper than 1

[Asgeir Ingvarsson]

API.Services.Models.Entities.Courses.Assessment.AssessmentGroup has a field OnlyIfHigherThan that references the ID of another AssessmentGroup. There should be a check to make sure that the referenced AssessmentGroup doesn't already have an ID in its OnlyIfHigherThan field so that we don't end up with a recursive, or even circular, definition.