devilry_student...test_groupdetails: Mock the grading system plugin.
devilry_student: Added template tag for rendering grade.
devilry_student...devilry_student_feedback: Removed unused template variable.
devilry_student.tests: Removed the tests for the old ExtJS UIs.
devilry_student.templatetags: Renamed devilry_student_feedback to devilry_student_tags.