graphql: upgrade grades type to class api

Test plan: specs pass

Change-Id: I28b45d7e2bccdbaae310094f4586a6e7bafa0fdd
Reviewed-on: https://gerrit.instructure.com/162848
Tested-by: Jenkins
Reviewed-by: Jonathan Featherstone <jfeatherstone@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>

app/graphql/types/grades_type.rb

@@ -17,19 +17,20 @@
 #
 
 module Types
-  GradesType = GraphQL::ObjectType.define do
-    name "Grades"
+  class GradesType < ApplicationObjectType
+    graphql_name "Grades"
 
     description "Contains grade information for a course or grading period"
 
-    field :currentScore, types.Float, <<-DESC, property: :current_score
-    The current score includes all graded assignments
+    field :current_score, Float, <<~DESC, null: true
+      The current score includes all graded assignments
     DESC
-    field :currentGrade, types.String, property: :current_grade
+    field :current_grade, String, null: true
 
-    field :finalScore, types.Float, <<-DESC, property: :final_score
-    The final score includes all assignments (ungraded assignments are counted as 0 points)
+    field :final_score, Float, <<~DESC, null: true
+      The final score includes all assignments
+      (ungraded assignments are counted as 0 points)
     DESC
-    field :finalGrade, types.String, property: :final_grade
+    field :final_grade, String, null: true
   end
 end

schema.graphql

@@ -453,11 +453,12 @@ enum EnrollmentWorkflowState {
 type Grades {
   currentGrade: String
 
-  #     The current score includes all graded assignments
+  # The current score includes all graded assignments
   currentScore: Float
   finalGrade: String
 
-  #     The final score includes all assignments (ungraded assignments are counted as 0 points)
+  # The final score includes all assignments
+  # (ungraded assignments are counted as 0 points)
   finalScore: Float
 }