System note was created as a discussion in MR

Description

In this MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23710) a system note was created as a discussion.

I would expect to not have a reply box under this note.

Steps to reproduce

Action	In commit page	In MR discussion tab	What should happen
Mentioning a commit in a comment on another commit	Creates a comment	Creates a broken system note that looks and works like a discussion. Also, creates a discussion with the comment text. Commenting on the discussion creates individual comments in the commit page.	Fix the broken system note. The comment in the commit page should not appear as a discussion in the MR page.
Mentioning a commit in a discussion on another commit	Creates a discussion	Creates a broken system note that looks like a discussion. Creates a discussion with the comment text.	Fix the broken system note.

Design