Introduce #reviewOptional as reviewState for non-impactful events on a subject

[foysalit]

Up until now, emitting events like modEventComment or modEventTag would set the reviewState of the subject to reviewOpen which is a bit disruptive to the moderation queue since these events are meant for setting optional metadata on the subject and should not require a moderator's review. This PR introduces a new reviewOptional state which will be set when such events are emitted on a subject with no prior (impactful) events.

TODO:
This means for events with type modEventLabel, we will no longer set the subject's status to reviewOpen so if tools like automod wants a labeled subject to be reviewd by a moderator, it will have to emit a separate modEventReport event.

[bnewbold]

I don't love the name #reviewOptional. Maybe #reviewNone or "empty" or something like that?

Automod already does label + create review in cases where that is needed or appropriate; it will also be good to be able to apply labels without requiring review. This is basically just parity with how image auto-labeling works (IIRC).

[foysalit]

I don't have strong opinion on the naming so happy to rename to reviewNone.
yeah, labels events will now only set to reviewNone if no previous state is set.

[devinivy]

This change stood out to me a little—just making sure we can account for the change in behavior.

[foysalit]

great catch! as discussed in slack, this was caused by a race condition and I missed the update in snapshot.
pushed a fix by adding forUpdate() to the current status selector query which should be safe since we shouldn't have that many concurrent events on the same subject.

[devinivy]

Had one question, but looks good!