Follow-up from issue #3027.

Motivation

On the pull request Activities tab, each update lists the commits from that push. To see build status for a commit, you currently have to open the project Commits page (or the commit itself). The PR header already shows jobs for the current head, but not for the other commits in the activity history.

Expected behavior

• Show the same overall commit build-status control used on the project Commits page next to each commit listed in PR Activities (including older updates and rebased commits).
• Place the icon beside the commit message (same place as on the Commits page).
• Clicking it should open the job list for that commit (same as Commits), in the context of this pull request.
• If the commit has no jobs, hide the control (same as Commits).

Scope notes

• Status meaning matches the Commits page: overall status of all jobs for that commit.
• Interaction matches the Commits page: overall status icon; click opens the job list (and can run jobs).
• Show the control on every commit listed in Activities, including older pushes and rebased commits (rebased rows are already marked as rebased).