FIX: when a flag is handled archive message for moderator group (PR #12798)

When a user flags a post with the “Something Else” option, a PM between the user and the moderators group is created. If no moderators reply to the PM, when the flag is handled at /review, an auto-reply is created for the PM. However, the PM is not archived, it stays in the inbox.

This commit ensures that the PM is archived for moderator group when no moderator has replied to that PM.

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