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: ![Screen_Shot_2019-06-13_at_8.12.44_AM](/uploads/0bfbb03d4c90a72fb4b8a127e8a1d116/Screen_Shot_2019-06-13_at_8.12.44_AM.png) 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: ![image](/uploads/75a5129a005b296d4f749ad48a18bb92/image.png) 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.
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