[Epic] Show deployment activity and current status for merge requests

Issues

• Show current deployment status of merge requests: #19571 (closed)
• Tie environments to external URLs: #19527 (closed)
• Show deployment date on merge request page: https://gitlab.com/gitlab-org/gitlab-ce/issues/21652
• Show deploy activity on merge request activity feed: #18440 (moved)

Links

• Might depend on #17013 (moved) to be able to display production deploys of a merged MR, although we can probably work around it if not available.