Show current deployment status of merge requests

Description including problem, use cases, benefits, and/or goals

Developers, Ops, PMs, and others should be able to quickly see if a merge request has been deployed, and if so, where it's deployed (dev, staging, prod), and quickly click through to said environment. This information should be in the activity stream (#18440 (moved)), but that isn't enough; current deployments should show up somewhere more prominently, in a fixed location.

Proposal

Use the word "Released" instead of "Deployed" though.

We can split this task up into two priorities:

1. Show current deploy status - Is this SHA currently deployed? - matching HEAD of branch with set of commits contained in each environment
2. Show date when first deployed - bisect set of deploys or keep track of records during each deploy (see #18440 (moved))

Links / references

• Part of #19992 (closed)
• Related: #18440 (moved)
• Benefited by: #19527 (closed)