Display merge commit SHA in merge widget after merge

Via https://gitlab.com/gitlab-org/gitlab-ce/issues/26723#note_60371603:

I want to add that the activity is often critically helpful for release managers. When picking MRs into stable branches, we'll look for the "So and so merged" and then "Mentioned in commit aabbccdd" system notes in order to know which merge commit to cherry-pick. I would be annoyed if these messages were hidden by default.

This would be a non-issue if we could update the merge widget after merging to display the SHA of the resulting merge commit. I'll open an issue for that.

Having that information front-and-center rather than buried in possibly hundreds of notes would help our release process a lot.

Did I add enough "merge"s in that title? 🤔

Design

• Add additional merge commit reference after branch
• Add copy to clipboard button to copy SHA

Note: Copy should be merged into master in 2e57be51