Re: "Relativise" pages during their build

[Chris Warburton]

The `render` function can now relativise, if asked to. This has a double
advantage: we relativise per-page (so the whole site doesn't need
updating at once), plus we're only invoking one Nix build process rather
than two.

I've tried to make the redirects do the same, but it doesn't seem to
work (many thanks to the test suite for spotting this though!). For now
it may be best to stick with two builds per redirect, but at least do
them per-page.