FIX: topic_creator accepts participant_count in import mode (PR #10632)

The issue mentioned here: https://meta.discourse.org/t/imported-private-discussion-doesnt-appear-in-the-author-inbox/163252

participant_count is important to attribute for private messages. If they are imported, we should allow them to set that attribute.

A workaround would be evaluating update_statistics method on each Topic but that is less performant.

GitHub

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https://meta.discourse.org/t/imported-private-discussion-doesnt-appear-in-the-author-inbox/163252/7