Make sure the topic_user.bookmarked column is set correctly when user bookmarks/unbookmarks any post in a topic.
- Previously this was only setting it to true if bookmarking the OP/topic, which was not correct – we want to show the icon on the topic list if any post is bookmarked.
- Also set to false if unbookmarking the last bookmarked post in the topic.
Also in this PR is a migration to correct any out of sync
topic_user.bookmarked columns, based on the new logic.