introduce `notificationCollapseKey` function to reaction message spec. For this method, I returned the joined result of the `type`, `threadID`, `creatorID`, and `targetMessageID` fields. By using these fields, a notification will be collapsed and won't alert the user if someone is quickly liking and then unliking the same message. I felt that this was reasonable, but can also see that we might want to remove `targetMessageID` from the joined results to make it less "spammy" if a user likes a bunch of messages in the same thread.
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Depends on D5870
Linear Task: [[ https://linear.app/comm/issue/ENG-2245/setup-message-reaction-spec | ENG-2245 ]]