ES: Protecting notes for confidential issue in search result page

https://gitlab.com/gitlab-org/gitlab-ee/issues/575

If it goes to the patch release, please include following to the release blog post:
--------------------------------------------------
If you use Elasticsearch please run following command after upgrade:

```
# Omnibus installations
sudo gitlab-rake gitlab:elastic:reindex_model MODEL=Note

# Installations from source
bundle exec rake gitlab:elastic:reindex_model MODEL=Note RAILS_ENV=production
```
----------------------------------------------------

See merge request !500