Add changelog file

Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9974, https://gitlab.com/gitlab-org/gitlab-ee/issues/9973, https://gitlab.com/gitlab-org/gitlab-ee/issues/9969

Improve the renderign of new and existing discussions
by reducing the number of watchers on each object & array.
Previously every discussion change would trigger an update for every
discussion component.

Also tidied up some components to get them closer to our docs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51506

The status is shown for
- The author of a commit when viewing a commit
- Notes on a commit (regular/diff)
- The user that triggered a pipeline when viewing a pipeline
- The author of a merge request when viewing a merge request
- The author of notes on a merge request (regular/diff)
- The author of an issue when viewing an issue
- The author of notes on an issue
- The author of a snippet when viewing a snippet
- The author of notes on a snippet
- A user's profile page
- The list of members of a group/user

Obviously not one of the best commits I always do.