For users that have the "mark new comments" feature enabled, this will
collapse old comments when they re-visit a topic that has new ones. It
involves adding a new "individual collapse" style that only collapses a
single comment and doesn't also hide all of its replies.
New comments and their direct parents will stay uncollapsed, and all
other comments in a path up to the root will be individually collapsed.
Any branches with no expanded comments will be fully collapsed. We
should probably add an indicator for how many comments are in a
collapsed chain so that we can distinguish between individually
collapsed ones and larger collapsed chains.