Threading scalability improvements

[grace-fagan]

To my knowledge this is how reddit handles threading overflow:

• On first load, shows max 10 layers of replies — top level post + 9 child comments
• On click of “Continue Thread”, shows next 10 replies (replaces content) with lowest reply highlighted at the top (shows siblings of lower level replies too)
• On click of “Show parent comments”, shows 10 parent comments with original comment highlighted (max 20 layers shown) (this seems like odd behavior to me)
• “View all comments” takes you back up to the top level (max 10 layers shown)