comment when auto-flagging as admin review (bug 1109701)

Fixes [bug 1109701](https://bugzilla.mozilla.org/show_bug.cgi?id=1109701)

This is following [bug 540028](https://bugzilla.mozilla.org/show_bug.cgi?id=540028) (PR #139) and [bug 1097634](https://bugzilla.mozilla.org/show_bug.cgi?id=1097634) (PR #363):
automatically add a review comment saying that the version has been flagged as "admin review" because some sources were provided.
Historically, when we've added automatic log entries like this (the examples I can think of are automatic compatibility bump and Jetpack repacks), the log entries have had the 'Mozilla' account as the user, rather than having no user at all. I don't really have a strong opinion on this, as long as you think creating records without a user won't cause problems elsewhere, but I think that would be my preferred solution.