Update merge request activity feed to include deployment to environment

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

As a developer, I want to know when merge requests get deployed (before and after merging)

Proposal

Split this up into two priorities:

1. Show activity for deploys that match exact SHA (this is relatively easy)
2. Show activity for deploys that contains SHA since last deploy (this may requiring searching all open and closed MRs)

Links / references

• Part of gitlab-ce#19992
• Requires Environments (gitlab-ce#17009)
• Made much more useful when we link pipelines before and after merges as in gitlab-ce#17013
• Benefited by gitlab-ce#19527 so we can make the activity linkable to the running environment
• Related: gitlab-ce#19571