mentionをテーブルに保存する
参考:
https://gitlab.com/gitlab-org/gitlab-ce/issues/22375
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12463
参考:
https://gitlab.com/gitlab-org/gitlab-ce/issues/22375
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12463