Currently `Topic#pm_topic_count` is a count of all personal messages tagged for a given tag. As a result, any user with access to PM tags can poll a sensitive tag to determine if a new personal message has been created using that tag even if the user does not have access to the personal message. We classify this as a minor leak in sensitive information.
With this commit, `Topic#pm_topic_count` is hidden from users by default unless the `display_personal_messages_tag_counts` site setting is enabled.
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