Add `mattermost_external_url` to gitlab.rb.template
https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-config-template/gitlab.rb.template does not appear to mention mattermost_external_url
anywhere.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-config-template/gitlab.rb.template does not appear to mention mattermost_external_url
anywhere.