Show CODEOWNERS section names in merge request approvals

Follow-up from #12137 (comment 326974068), where CODEOWNERS sections were first introduced. With that issue, we're showing the “sectioned” CODEOWNERS approval rules in MRs, but not showing the actual section name. For further context on the problem, see that issue.

Proposal

In the merge request widget, when listing CODEOWNERS approval rules:

1. For entries outside of sections, we keep displaying them as today.
2. For section entries, we display the name of the section above the approval rule, in a smaller, muted text style (if it helps, I used the classes small and text-muted in the image below).