Instead of going to the OP of the topic for topic-level bookmarks when clicking on the bookmark in the quick access menu or on the user bookmark list, this commit takes the user to the last unread post in the topic instead. This should be generally more useful than landing on the unchanging OP.
To make this work nicely, I needed to add the last_read_post_number to the BookmarkQuery based on the TopicUser association. It should not add too much extra weight to the query, because it is limited to the user that we are fetching bookmarks for.
Also fixed an issue where the bookmark serializer highest_post_number was not taking into account whether the user was staff, which is when we should use highest_staff_post_number instead.