Use counter cache on Courses#show

Author: nygrenh

app/controllers/courses_controller.rb

@@ -203,7 +203,11 @@ def assign_show_view_vars
         @submissions = @course.submissions
         @submissions = @submissions.where(user_id: current_user.id) unless can? :teach, @course
         @submissions = @submissions.order('created_at DESC').includes(:user)
-        @total_submissions = @submissions.count
+        @total_submissions = if can?(:teach, @course)
+          @course.submissions_count
+        else
+          @submissions.count
+        end
         @submissions = @submissions.limit(max_submissions)
         Submission.eager_load_exercises(@submissions)
       end