fix: Read status doesn't update after adding a new comment

[taimoor-ahmed-1]

This PR fixes the mysql api for adding a new comment where the thread wasn't getting unread due to last_activity_at not being updated

close #219

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[Faraz32123]

can we do it like this?? As we already have comment_thread above.

if comment_thread:
    comment_thread.last_activity_at = timezone.now()
    comment_thread.save(update_fields=["last_activity_at"])