Add deployment environment names

See https://docs.gitlab.com/ee/ci/environments/

This enables GitLab to

• display some nice extra links with every deploy that we can click on to check out the new deployment after it's done
• allow us to trigger a deployment of an older version (rollback) if a new one is garbage
• later hopefully: together with reading metrics from Prometheus allow auto-rollback

For this to work, @jcalderone please set up two environments as per the GitLab docs here:

In the UI:

Go to the project’s Operations > Environments page. Select New environment. Enter a name and external URL. Select Save.