devilry_subjectadmin.rest.assignment: Include has_valid_grading_setup.
devilry_subjectadmin.rest.assignment: Removed gradeeditor field, and added

gradingsystemplugin_title.
devilry_subjectadmin...model.Assignment: Remove gradeeditor and added

the new has_valid_grading_setup and gradingsystemplugin_title.
devilry_subjectadmin...EditGradeEditor: Updated for devilry_gradingsystem.