Show link to a whole discussion from discussion tab to diff

From https://gitlab.com/gitlab-org/gitlab-ce/issues/48169

We have links to individual comments in a discussion but we don't show any single link to the whole discussion. One possible solution I can think of is including the link to the first comment in the system note summary line.

@username started [a discussion](link-to-the-first-comment) on [the diff](link-to-diff)