Count notes for commits and merge requests

What does this MR do?

This is a part in https://gitlab.com/gitlab-org/gitlab-ce/issues/64298 it counts notes on merge requests and commits.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content