Extract discussion notes into new component

What does this MR do?

Merge Request on CE

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27066

Issue on CE

https://gitlab.com/gitlab-org/gitlab-ce/issues/58294

Original description

This is a part of the work to split NoteableDiscussion component into smaller components: https://gitlab.com/gitlab-org/gitlab-ce/issues/55566

Discussion notes have been moved out of NoteableDiscussion into a dedicated DiscussionNotes component:

The goal is to have a better separation of concerns in this part of the app, which should result in easier to maintain components and fewer regressions in the future.

Why is this backport necessary?

GitLab EE includes an additional DraftNote component in the discussion notes. It was necessary to add a dedicated slot in the new DiscussionNotes to preserve this draft feature.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers