Backport CE changes for: [Frontend only] Batch comments on merge requests

What does this MR do?

Backports CE changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7376

What are the relevant issue numbers?

See: https://gitlab.com/gitlab-org/gitlab-ee/issues/1984

(this doesn't close it)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides