This can later be loaded from JSON or something, but for now it just
lives with the rest of the code. I'm already going to mix data with code
in this repo and these values shouldn't change much, so I'm fine with
that.
This is my personal site after all. I'm not expecting to release this in
a way for others to use.
Detects whether there's a previous page or next page, the old HistoryNav
was made to be dumber and most of the logic now lives in Posts (which is
the only thing that uses HistoryNav anyway)
The side-effect is that the nav is now within the content column, which
is okay since it's symmetric now.
