Fix n+1 query in gradebook history feed endpoint

Closes PLAT-2686

Test Plan:
- As a student create submissions for several assignments
- Retrieve gradebook history via the api for that student:
  /api/v1/courses/<id>/gradebook_history/feed?user_id=<id>
- Verify that an n+1 query does not occure for originality
  reports. (the repeating query we want to avoid look something like this:
  SQL OriginalityReport Load (4.1ms)  SELECT "originality_reports"
  .* FROM "originality_reports" WHERE "originality_reports"
  ."submission_id" = 55487625)

Change-Id: Id5ba23772bc3eaa3b7ac253b4e6c61a8744ed6b3
Reviewed-on: https://gerrit.instructure.com/116270
Tested-by: Jenkins
Reviewed-by: Andrew Butterfield <abutterfield@instructure.com>
Reviewed-by: Keith T. Garner <kgarner@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Weston Dransfield <wdransfield@instructure.com>

lib/api/v1/gradebook_history.rb

@@ -67,6 +67,7 @@ module Api::V1
       unless opts[:submission]
         ActiveRecord::Associations::Preloader.new.preload(versions, :versionable)
         submissions = versions.map(&:versionable)
+        ActiveRecord::Associations::Preloader.new.preload(versions.map(&:model), :originality_reports)
         ActiveRecord::Associations::Preloader.new.preload(submissions, :assignment) unless opts[:assignment]
         ActiveRecord::Associations::Preloader.new.preload(submissions, :user) unless opts[:student]
         ActiveRecord::Associations::Preloader.new.preload(submissions, :grader)

spec/apis/v1/gradebook_history_spec.rb

@@ -135,6 +135,31 @@ describe Api::V1::GradebookHistory do
     end
   end
 
+  describe '#versions_json' do
+      let(:grader) { User.create!(:name => 'grader') }
+      let(:student) { User.create! }
+      let(:assignment) { course.assignments.create!(:title => "some assignment") }
+      let(:versions) { [Version.create!(versionable: submission, model: submission)] }
+      let(:harness) {  GradebookHistoryHarness.new }
+      let(:submission) do
+        s = assignment.submit_homework(student)
+        s.update_attributes(graded_at: now, score: 90, grade: '90', grader: grader)
+        s
+      end
+      let(:course) do
+        c = Course.create!
+        c.enroll_student(student)
+        c.enroll_teacher(grader)
+        c
+      end
+
+    it "does preloads originality reports" do
+      submission.reload
+      harness.versions_json(course, versions, api_context)
+      expect(versions.first.model.association(:originality_reports).loaded?).to eq true
+    end
+  end
+
   describe '#submissions_for' do
     before :once do
       @course = Course.create!