ChatOps integration for demo

This issue tries to discuss possible approaches for integrating with ChatOps for purposes of a demo: https://gitlab.com/gitlab-org/gitlab-ce/issues/22190

There's a lot of material already written:

• Slack slash commands: https://gitlab.com/gitlab-org/gitlab-ce/issues/22182
• Slack app: https://gitlab.com/gitlab-org/gitlab-ce/issues/20799
• Mattermost slash commands: https://gitlab.com/gitlab-org/gitlab-ce/issues/22540

There's also a Slack/Mattermost slash commands implemented already: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6400

We decided that we should use Mattermost for demo purposes.