Broken service desk emails on GitLab.com

Reported in https://gitlab.com/gitlab-org/gitlab-ce/issues/55104

not only Service Desk emails but also all the emails that are using mail_answer_thread (in app/mailers/emails/issues.rb and app/mailers/emails/merge_requests.rb).

Patch proposed in https://ops.gitlab.net/gitlab-com/gl-infra/patcher/merge_requests/19