Backfill threads

Currently in the extension. The problem is when you move on to the last page. There might only be a couple of posts which makes it difficult to put it into context.

Personally, I'd be tempted to just automatically have a minimum of 5 posts on the last page (and perhaps style those imported ones differently). Maybe game threads only.

e.g. with 22 posts in a thread.
Page 1: show posts 1-20
Page 2: show posts 18-22

This is an interesting one. It goes against a typical forum setup, and almost leads to a lazy load system, which may be a better idea, needing some JS and a new endpoint to get posts within a range.