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, 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.