Use extraFields in resource classes

Following the design guidelines of Yii, the resources classes should not have data members containing redundant information, which is otherwise available through navigational properties; but the extraFields list should contain them instead an be expandable via the expand query parameter.

E.g. the courseName, courseID, groupNumber fields come from the group object of the ExamTestResource, but they should be removed and the group should be in the extraFields instead.