Use unordered Course.assessments to get categories (autolab#1962)

Course.assessment_categories needs to make sure that the associated
.assessments do not have an .order scope attached

(cherry picked from commit 286fb0a)

app/models/course.rb

@@ -254,7 +254,7 @@ def instructors
   end
 
   def assessment_categories
-    assessments.distinct.pluck(:category_name).sort
+    assessments.unscope(:order).distinct.pluck(:category_name).sort
   end
 
   def assessments_with_category(cat_name, is_student = false)