Allow system notes to _not_ be associated with a user
## Summary
Currently, all system notes are associate with a user and this user is displayed in the UI:

However, there are cases in which it may not make sense for a user to be associated or displayed on a note. For example, as part of the [merge trains feature](https://gitlab.com/gitlab-org/gitlab-ee/issues/9186), a merge request can be automatically removed from the merge train by the system:

In this case, this message is misleading; the action was initiated by the **system**, not the user.
## Summary
Make it possible to create a system note that is **not** associated to a user.
issue