[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.

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.