DEV: Added commit approved notification type for discourse-code-review

We need to reserve ids in core so plugins do not clash.

app/models/notification.rb

@@ -63,7 +63,8 @@ class Notification < ActiveRecord::Base
                         watching_first_post: 17,
                         topic_reminder: 18,
                         liked_consolidated: 19,
-                        post_approved: 20
+                        post_approved: 20,
+                        code_review_commit_approved: 21
                        )
   end