course: last_reg_update field in API for hourglass

Author: kylesferrazza

app/controllers/api/courses_controller.rb

@@ -19,7 +19,8 @@ def serialize_course(course)
       {
         id: course.id,
         name: course.name,
-        prof: current_user.course_professor?(course)
+        prof: current_user.course_professor?(course),
+        last_reg_update: course.last_reg_update.iso8601,
       }
     end
   end

app/models/course.rb

@@ -340,4 +340,8 @@ def unpublished_grades_query
       .order("users.name")
       .select("DISTINCT submissions.*", "users.name AS user_name")
   end
+
+  def last_reg_update
+    course.registrations.order(updated_at: :desc).first.updated_at
+  end
 end