Add settings and templates for Reply by email
See gitlab-org/gitlab-ce!1173 and https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/reply_by_email/README.md
cc @marin
See gitlab-org/gitlab-ce!1173 and https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/reply_by_email/README.md
cc @marin