In the user bookmark list, when we show the excerpt of the bookmark
(which is usually just the bookmarked post excerpt), we want to show
the first unread post's excerpt instead for for_topic bookmarks. This
is because when the user clicks on that bookmark link, they are taken
to the first unread post in the topic, not the OP, as per:
27699648ef
Instead of going to the OP of the topic for topic-level bookmarks
(which are bookmarks where for_topic is true) 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.
We don't need no stinkin' denormalization! This commit ignores
the topic_id column on bookmarks, to be deleted at a later date.
We don't really need this column and it's better to rely on the
post.topic_id as the canonical topic_id for bookmarks, then we
don't need to remember to update both columns if the bookmarked
post moves to another topic.
