Handle differently _system_ note

[fcollonval]

GitLab is listing system note (label changes, new commits,...) when requesting the comments on a pull request:

https://docs.gitlab.com/ee/api/discussions.html#list-project-merge-request-discussion-items

It would be good to display them as read-only information as GitHub and GitLab do.

It would be nice if we can get those information from GitHub too.